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Before I even get into this:

I'm aware that Shuuichi Asou (mangaka) is a SaiTeru shipper; it doesn't mean anything to me that he included SaiTeru title cards in S2. He was sharing his art of his characters and he continues to do so, as an artist does. If SaiTeru was his goal for the series he would have made it canon, but he did not. Saiki says "I don't want either of them (Aiura and Teruhashi)," when his grandparents find out about them and it's left on that note.

The show is a gag meant to make fun of tropes. Teruhashi is the parody of the "perfect girl," usually the first girl introduced, that's flawless and always ends up with the underdog mc regardless if some other girl that's more fitting comes along. Except here, Saiki is overpowered and unintentionally popular and Teruhashi is so perfect that when Saiki ignores her existence, she thinks she's the one in love. It's meant to be ridiculous.

Getting into it:

I can't say I blame him. He doesn't feel attracted to walking muscle bags.

He doesn't want a relationship because of his powers. He says that all the way to the end and he sticks to that. If he didn't have his powers it might be a different story.

Romantic feelings aren't completely based on looks; feelings can develop overtime regardless of physical attraction.

He consumes romantic media; one of the first things he's shown trying to watch on TV is "Love Fantasy" until it gets spoiled. He also was very invested in those stranger's love story when he accidentally touched that book. So he is at least ok with media. Left at that, aego is fitting.

Aegoromantic is defined by a disconnection between oneself and the subject of romantic attraction. They may enjoy the depiction of romance in media but feel little to no desire to participate in romantic relationships themselves.

He is also supportive of his friends being in love, as long as that "love" isn't directed at him. He wants Kaidou and Yumehara to get together, so Yumehara doesn't set her sights on him again.

Where you may disagree is his feelings for Teruhashi, Aiura or Satou. His feelings are platonic for all of them.

Satou is the first person he's expressed wanting a friendship with; he likes how average he is and tries to impress Satou with having the same interests. Saiki stalks all his friends btw; the stalking was just blatantly shown for Satou to prove a point. It's a squish.

A "squish" is the platonic counterpart to a "crush." It describes an intense desire to be close to someone or form a deep friendship, but without any romantic or sexual interest.

Saiki has a very good relationship with Aiura because she understands him. She talks to and jokes around with him and makes him feel included. They are best friends and he feels comfortable enough to hug her, and she is often his go to when he needs help.

He realizes as time goes on that Teruhashi has started to view him as a human being, starting at the mixer; she saw him as any other guy before so that was shocking to him and he starts to consider her as a friend rather than a girl who is after him. He is impressed by her efforts in being perfect, and he wishes she would just be herself and not worry about what other people think of her. He is concerned for her wellbeing. In reawakened, he blushed at her; it's not because he likes her. He was objectively analyzing her, as he does all things, and she caught him staring. He's embarrassed.

If I saw any of that as being romantic then I'd say he's demiromantic.

A demiromantic person only experiences romantic attraction after forming a deep emotional bond with someone.

There's a 50% chance he is aromantic, but he is arospec.

3. Gender

Because you brought it up.

Saiki was born, in a transformed state, as female. So trans, technically. He isn't sure which form is his true form since something he struggles with is control over his powers, however:

His default state seems to be male, and if there's anything to take from this panel he was assigned male at birth. He also goes by he/him pronouns.

Saiki is an opportunist. If a situation calls for it, he will turn into a girl, and he has said he'd want to be a cute girl in his next life. He cannot be a girl indefinitely as that power only lasts for 2 hours, and need 5 hours before transforming again..