Two points:

Point A. Remember what I said in my analysis of Episode 11 that I disagreed with the general interpretation that when Jiaxu muttered, “What am I thinking?” and “Kiddo,” he had been appalled with himself for developing inappropriate feelings for her because she was still a kid? I said that was the WRONG interpretation because it DID not align with the events in Episode 11 and in Episode 12.

To recap:

Beginning Episode 8, when they met again, he already knew that she was no longer a minor. She repeatedly told him that she was an adult. And he accepted it although he had a moment of resistance when he realized that she could smoke if she wanted to, drink with whomever she wanted, and wear make-up as she pleased. He was hoisted by his own petard! Back in Episode 6, while he was scolding her about her online boyfriend, he said, “Once you’re in college, you can do whatever you want. No one will stop you. Do you hear me?”

To counter his unfavorable impression of her on their reunion, Sang Zhi dressed to impress him at their dinner date. Like a well-brought young lady, she wore a pretty pink dress and matched it with sneakers. Unfortunately, her plan to impress him fell through when he got sick with appendicitis in Episode 9. She rushed him to the hospital and there she managed to impress him with something else: her patient care.

Remember what he said when she did her best impression of a boss lady?

SZ: What’s wrong?
DJ: Hearing your lecture me makes me realize that our Sang Zhi has grown up.
SZ: (flexing) I am grown up. I’m not a kid anymore. Did you just realize that? I’m off.

But Jiaxu wasn’t the only one impressed with her. His elderly roommate so admired Sang Zhi’s bedside manners that he deemed her marriage material and badgered Jiaxu to marry her quickly. To her extreme annoyance, Jiaxu merely brushed off the man’s banter as a joke.

Episode 10 revolved around this marriage joke. Unknown to Jiaxu, Sang Zhi was offended by his participation in the joke. She assumed that he had meimei-zoned her, and grew more and more despondent, believing that there was nothing she could do to make him see her as an adult. But unknown to her, Jiaxu also felt like he’d been Gege-zoned. When she joked that he was too old to be her partner, he concluded that there was nothing he could do to make her see him as a boyfriend. Thus, he resigned himself to simply being her Gege…which is precisely what Sang Zhi didn’t want.

Episode 11 saw the evolution of his feelings for her. I said this already –

During the New Year’s Eve dinner, he was determined to keep his feelings hidden, and act like the Gege HE THOUGHT she wanted him to be. He Gege-zoned himself because he didn’t want to hurt her feelings again by joking about marriage.

After the water slap, and when she nursed his bruise, his feelings for her nearly overwhelmed him once again. He wanted to touch her hair. But he restrained himself because he didn’t want to fluster her like she was flustered when they accidentally kissed. When the first snow fell, he began to hope that things could become better for them. As long as she was happy and healthy, so would he be. After he went home and encountered the psycho who water-slapped him, he found new courage from Sang Zhi’s words to cut ties with his past. He knew he had already fulfilled his financial obligation; he was no longer willing to carry the punishment forever.

As he watched the fireworks with her on the other side of town, he knew that Sang Zhi was key in altering his mindset about his future. He wanted to change her name to a more familiar one, Zhizhi, which would reflect the way he felt about her. But he hesitated because he knew that he still had his work cut out for him. In his mind, Sang Zhi had pigeonhole as a platonic friend. Shifting this view of him to a romantic one couldn’t be rushed. But he had time and patience on his side.

That’s why I say that Qian Fei’s brief phone call couldn’t make him do a volte-face. From Episodes 8 to 11, he’d been steadily forming an awareness that Sang Zhi had blossomed into a lovely and capable adult. So why would he flip-flop and revert to viewing Sang Zhi as a “kiddo” at Qian Fei’s surprise call?

It’s not logical.

Calling the absent Sang Zhi a “kiddo” is a rhetorical device. It’s called antiphrasis. It’s inverting the meaning of the word or phrase. For example, saying, “Just great!” when you spill coffee on your blouse on the way out of the door for an 8am meeting. Or calling your 6’5 athletic husband, “Hey, baby.”

Jiaxu used the word “kiddo” to mean the exact opposite of its literal meaning. He obviously no longer thought of her as a kid. He was showing indulgence and fondness in a brand new way.

That’s why I said the general interpretation of this scene is wrong. It didn’t take into account the events of the past episodes.

Point B. In addition, I think that the many viewers didn’t see that their interpretation is inconsistent with the events of Episode 12.

Here’s what happened in Episode 12: the airplane. It was the featured object in this episode.

Here are my three interpretations of the airplane.

1. The first was pointed out by @agdr03. 

She posted:

Did the airplane refer to DJ having the courage to fly towards SZ? Before he dropped her to the airport he was already flying towards her.
When he flew back to Nanwu. He was determined then. He knew he’s flight course. 😄

I replied:

Yes, that’s WHAT I THINK the airplane was for. It symbolized his flight plan, his trajectory, his intended flight towards her.
And it balances/counters the paper airplane.

Good work, @agdr03!

2. The second interpretation is protracted.

If we follow their dialogues in this episode, it was Jiaxu who constantly reminded her of her flight back to Nanwu. WHY?

a. Sang Zhi was waiting for Jiaxu to call her up and explain his abrupt demand not to visit him at his office or home without giving him heads-up. She was listless and only perked up when he finally texted her. Notice that he texted her after supervising his colleague’s work.

DJ: When are your exams? Have you bought you plane ticket home for the school break?
SZ: Next week. I bought a ticket for the 19th.
DJ: Okay.

She was a little bit disappointed by his one-word answer. Huh? That’s it?? She didn’t know that he was jotting down the day so he’d remember it. He was going to take time off from his busy work schedule to take her to the airport and see her off.

She was relieved when he texted her again.

DJ: When will you be free after your exams? Let’s have a meal together.

She lit up at his text. She decided to play coy, however.

SZ: I might not have time.
DJ: (sighing and mentally thinking) She might go out with friends. Forget it. (then texting her) Okay.

Of course, he was disappointed. But remember what I said about his resolution after that Qian Fei phone call? He resolved to bide his time, without pressuring or bullying her. As usual, Sang Zhi took it the wrong way. She assumed that he wasn’t really interested in eating out together and that his offer was perfunctory. She was depressed all over again.

b. Then, on the eve of her flight back to Nanwu, he texted her again. Notice that he texted her while he was in the middle of work. Despite his heavy workload (he was working late while they were partying), he was worrying about her.

DJ: (texting) You’re going home tomorrow. Have you packed your luggage?
SJ: (no response because she was at the bar)
DJ: (texting) I’ll send you to the airport.

Note: He didn’t ask permission to take her to the airport. He simply declared it, expecting no arguments from her.

SZ: (ignoring his text)

Because he was expecting her eyes to be glued to her phone, he called her up when she didn’t respond to her Lord and Master. (Just kidding. He could probably detect that she’d read the messages. Since she didn’t reply, he wondered what’s up.)

SZ: Hello?
DJ: Why didn’t you reply to my messages?
SZ: (no response)
DJ: Hello?
SZ: (no response)

Through the phone, he heard the swelling music in the bar. Poor Jiang Ming! He intended his song for Sang Zhi’s ears, not her Gege’s.

DJ: (sternly) Where are you?
SZ: I’m at a bar.
DJ: Who are you with?
SZ: My friends.
DJ: (inhaling) Did you drink?
SZ: Just a little.

Note: He doesn’t like Sang Zhi to drink. Sure, there’s his family history to consider here. Based on his experience, no good things happen to those drink. But it’s interesting that he doesn’t mind Sang Yan getting drunk as a skunk, but he won’t tolerate Sang Zhi even slightly tipsy.

Note: as a public service announcement, here’s my list of Do’s and Don’ts on partying where there’s free booze.

DJ: What’s the name of the bar?
SZ: (names the bar)
DJ: Okay. Wait outside. I’m coming over now.
SZ: Why are you coming?
DJ: To catch a drunkard. (hangs up)
SZ: Hello? (talking to the phone) You won’t let me go and see you. But you want to come and see me. You’re so weird.

Note: Sang Zhi talking to the phone, like Jiaxi flicking Sang Zhi’s memo card, are their way of expressing emotions they couldn’t directly say to each other. If they could, they would. But since they didn’t have the courage, they had to release their emotions in alternative ways, like talking to inanimate objects.

c. The following morning, Sang Zhi woke up shortly before 9am and remembered fragments of what happened the night before. She was mortified. Then, Jiaxu began sending a spate of texts.

DJ: Are you up? I asked your roommate to wake you up at nine.
DJ: All the flights at this time are sold out. You can’t change your flight. You have to get up or you won’t make it home.
DJ: Drink a lot of water after you wake up. I’ll pick you at 10 and take you to the airport.

d. When she walked out the dorm, he was waiting for her in the car.

He helped her with her luggage and looked eager to spend time with her on the drive to the airport. Unfortunately, however, Jiang Ming showed up with a beef burger from McDonalds, and that ruined the whole trip for them. He had to eat the beef burger and pretend to like it. It didn’t matter if he were to choke to death on it, there was no way was he going to let her eat it. She was allergic to beef and, more importantly, he didn’t like the idea of her eating food given by another guy.

SZ: Are you really that hungry?
DJ: No.

The scene had me laughing because I’m reminded of croissants. Do you know the origin of croissants?

Legend has it that the Viennese bakers invented the croissant in 1683. The Austrian capital had been under siege by the Ottoman troops. They were digging a tunnel to bypass the walls around the city. But the bakers, who were baking in the cellars, heard their noise, and alerted the Austrian soldiers. The Austrians went on to repulse the invaders and to celebrate their victory, the bakers invented a bread in the shape of the crescent. You see, the crescent was an emblem of the Ottomans. Supposedly then, when one takes a bite of the crescent-shaped bread, he is also taking a bite of the enemy.

And that’s why I’m reminded of croissants when I saw Jiaxu chomping down on the beef burger. He could imagine that he was biting Jiang Ming, limb by limb. He knew that the young man mistook him for a blood relative of Sang Zhi. It must have annoyed him not to correct it, but he let it go. He had to pretend that he didn’t mind the guy talking to Sang Zhi, even though he suspected him of getting Sang Zhi drunk twice in a row. He gave them permission to talk and even returned Jiang Ming’s wave of goodbye with a smile.

The ride to the airport turned out awkward for both of them. In my opinion, he had planned to have a comfortable ride with her and to begin his slow campaign to get her to change her perception of him as an old, boring brother. Instead, it became an interrogation session for her.

So…going back to Jiaxu’s phone conversation with Qian Fei, discerning viewers should realize by now that the general interpretation of his feelings back then does not fit the narrative. Jiaxu did NOT disavow his feelings for her because a) he still thought of her as a “kiddo” and b) he worried about the psycho girl. Nope. It’s not consistent with what I’ve highlighted here.

By now, discerning viewers should also begin to wonder why screenwriter/director had to deviate from the web novel and a) feature the airplane as the object of interest in this episode, and b) spotlight Jiaxu checking up on Sang Zhi’s flight.

Well, I’ll give you two reasons.

First, it’s to show how different Sang Zhi’s arrival to YiHe was from her departure.

Jiaxu knew that she had come all the way there by herself. In a bid to show her independence, she moved without her parents’ or her brother’s help.

Jiaxu was tacitly encouraging her to rely on him because he would take care of her better. He wouldn’t allow her to go on a long journey unaided.

Second, it’s to show how different Gege #2’s treatment of her was from her real Gege, Sang Yan. In Episode 13, Sang Yan picked her up from the airport, but he was thirty minutes late because he went to buy her favorite food. He was expecting brownie points, but I thought their parents were right to scold him. He SHOULD have anticipated a line and gone earlier.

And he couldn’t complain about the trouble he had getting her food in the afternoon when the poor suitor Jiang Ming woke up early in morning to go to McDonalds and buy burger for breakfast for Sang Zhi. That’s what a typical admirer would do: go out of his way to do thoughtful things.

But let’s look at what Jiaxu did for Sang Zhi. Two weeks before her departure, he reminded her to buy tickets. Then, on the eve of her departure, he reminded her to pack her clothes. Then, because she was drunk, he reminded her friends to wake her up at 9am. To be sure, he texted her a reminder that he would pick her up at 10am. Inside the car, he had fruits ready (I bet he cut them up himself) for her hangover, and he planned to buy her a hot drink at the airport.

Jiaxu went over and beyond what a real Gege and a typical admirer would do for her.

3. My third and last interpretation is visual.

When Jiaxu gave her a piggyback ride, he was transporting her from one place to another, like an aircraft. Lol.

There you go, kiddos.  Next, Episode 13.