Right at the start, you should say "don't mess with me" (in case you're an emo)/"I'm here to have a good time" (which can also be taken positively).
Get on his good side via Ragata (if you approach him directly, you’ll have to avoid warning Gangle about the prank, even though it’s obvious, so that could count as rude), in their argument say “I can enjoy a good prank”, agree to his “tour”, and when you’re dragged after to look for sentipid with no choice, you can tell Ragata straight out that Jax dragged you in by force. Ooooor you could pull a Looney-Tunes prank by throwing the sentipid straight at Ragata – cruel, of course, but the protagonist justifies it by saying they simply panicked, so the whole thing looks more like a funny situation than a direct prank on Ragata
Then EITHER you can say straight out that he was the one who made us bring the omelette, but after that, even if they force us into a prank on Gangle as a distraction, it’s worth covering for Jax with “he was just passing by”, OR say that you decided to bring the omelette yourself (...technically true, the protagonist didn’t refuse), and you can hand Jax over to Cain, and then from there on it seems you can behave yourself everywhere
I ended up going for him even before I actually sat down with him face-to-face, because I didn’t want to start off on a rough note and I saw the possibility of alternative routes lol. In this scenario, it seems that the protagonist is that very same nice character who was assigned to the bully as a victim, and just started trying to fix him (and even succeeded a little) xD