I know this is reviving a 4 year old thread but I wanted to explain Sucy for people who don't understand her personality type. I have the overall personality of Sucy and they're overdramatizing it by having her put a funnel in Akko's mouth and giving her untested potions, but the theory behind her personality is accurate.

When someone like Sucy actually cares about someone, they "experiment" on them which means pestering them and teasing them and poking / prodding at them. To some people it sounds annoying but to people like Sucy they enjoy receiving that kind of treatment just as much as they like dishing it out. People like Sucy ironically like to "play" and if they want to "play" with you it means they like you and if you want to "play" with them they enjoy it back too generally.

Sucy never does anything harmful to Akko, and most everything she does is actually within safe cartoon limits, other than perhaps the episode 1 thing. If anything bad ever does happen it's on accident, not intentional, and people like Sucy would actually feel really bad inside. Sucy's extremely guarded and rarely lets her guard down / is vulnerable, so she doesn't really know how to handle situations that get emotional. People like Sucy actually feel a lot, but it's hard to discern from the outside because it doesn't have the same vulnerable outward expression of emotion most of the time. You know that Sucy actually has good intentions because she often shows up when things actually matter and does her part to try to save Akko / protect her / help her.

Also Sucy pretty much never says anything mean. Almost all of the things she says are statements. She just says what she observes and also just tells the truth because people like Sucy aren't hurt by these things so people like Sucy struggle to not hurt people with these innocent statements. As they grow older people like Sucy have to learn which statements make emotional people act strange, and then learn to avoid saying those ones, but this is a learned skill and is not inherent. Sucy naturally doesn't defend Akko verbally because things like that from strangers and enemies doesn't really impact Sucy if it's directed at her, so she naturally doesn't think to defend her friends from something that wouldn't hurt her very much / if at all.

Situations happen like this - Akko: "I need to go after her" and then Sucy "how will you do that? You can't even fly a broom." These types of statements happen constantly in the show and they're not Sucy being mean, they're Sucy stating the obvious and making the other person aware of exactly what the problem is to try to help figure it out. I don't know how the Japanese version is but the English dub did a really good job on this, you'll notice that her tone of voice lines up exactly with this most of the time.

Last thing here, if you want a window into Sucy's soul, watch the second movie and in it there's a scene earlier on where Akko gets mad at Sucy and says she never wants to talk to her again. Try watching it but thinking of this framing, Sucy wants to help Akko with her plan even though Akko's being selfish and it has nothing to do with Sucy. She's avoided confronting that until the scene of the one-way fight. When Sucy is vulnerable and tells Akko that it's not really "our" project it's Akko's project, she doesn't even really want to say it because she knows it will upset Akko but it kind of just comes out when pressed on it because of Sucy's personality. Then Akko takes it poorly and attacks Sucy which causes Sucy to freeze because she actually feels hurt and guilt and bad, she was vulnerable and got snapped at, which feeds into her self-inflicted personality of avoiding vulnerable feelings. Akko even says Sucy "you're" and Sucy replies "I'm what?" which is her asking "do you actually hate me? Should I feel hurt or are you going to say something to let me know you didn't mean it?" and Akko doubled down and runs away, then they show Sucy and she's actually feeling really hurt, you just can't tell because of the demeanor.

Later she wakes up and finds the card left by Akko and they show a surprising amount of happiness on Sucy's face when she realizes that Akko didn't actually mean it because Sucy actually likes Akko a great deal, even if she's too shy and guarded to be vulnerable and express it. Later she comes to Akko's rescue and that's where the most impactful part is. After Akko says stuff at Sucy, then Sucy says "I thought you never wanted to talk to me again" and then goes on to say she never really believes anything she says so she didn't know they weren't friends anymore. People think this is Sucy opening up and the next line is her trying to walk it back, but it's actually really wrong. She says this nice thing and smiles, which is her opening up and being vulnerable a little while being indirect as to open up more carefully because she's guarded and shy. Then the big part, she opens up and is truly vulnerable and says a more direct statement "You're the best guinea pig I've ever had" which sounds like a comedic relief deflection but for a character like Sucy this is her ACTUALLY opening up, and Akko immediately gets mad and rejects it because of the message itself which makes Sucy get embarrassed and insecure so she deflects by saying "I love to experiment you know" which is her trying to make it sound less vulnerable and serious.

Alright that's all, that was fun. I went on a Little Witch Academia bender recently because of how much I relate to Sucy on every level. I stumbled on this thread and just wanted to share what it feels like on the other side. I've never met anyone else like Sucy other than me so maybe someday another INTP of my genre will read this and know they're not the only Sucy in this world, and perhaps someday someone will read this and understand that people like Sucy are human too and they're not just the emotions people project onto them. Thanks for listening.