Well, in 'Finn', the characters are made memorable through their relationships with each other. Finn's interactions with his friends or foes create interesting dynamics. For example, if he has a best friend who is always getting him into trouble but also helping him out of it, that kind of relationship makes both characters stand out. Also, the character development throughout the story is key. A character who starts off as shy and then becomes bold over time is very memorable.
The Unti Finn novel by A. J. Finn is a complex mystery that weaves together multiple plotlines and keeps the reader guessing until the very end. It's full of unexpected twists and turns.
The Fire Kingdom's Flame Princess could be an ally too. Her mastery over fire could complement Finn's powers in combat situations. Also, the various magical creatures in Ooo, like the gnomes or the fairies, could band together and support Finn. They might have their own ancient knowledge or magic that could be useful to Finn in his adventures with his godlike powers. Finn's kindness towards them in the past might make them more inclined to help him out.
He might meet some of the more reclusive characters in Ooo that we don't usually see in the show. Maybe a hermit - like wizard who gives him some cryptic advice.
Kurt could be a great best friend for Finn in the fanfiction. They have different personalities but could learn a lot from each other. Kurt could introduce Finn to new styles of music and performance, and Finn could help Kurt be more confident in his own abilities. They could bond over their shared love for the Glee Club.
Finn's personality would probably become more brooding. The whole vampire thing is a big change, and he might start to withdraw a bit. He was always so full of energy and positive before, but now he could be more cautious and a bit more on the dark side. He might also become more territorial, like vampires tend to be.