I rarely develop real long term attachment to characters, but the few I really enjoyed have been Solid Snake, Big Boss, and Ezio Auditore mostly because we really watch their lives unfold and I must say I like a sequel showing older versions of characters getting on in years. The scenes with a 80 year Altair in Revelations are quite well done from both a story and gameplay standpoint.

From Final Fantasy, there are a few like Vivi who really stand out. I also have to bro-fist Del and say that Crono has always been a character I've been really attached to. I think his death midway through the game really cemented those feelings, but he also just has some funny quirks that keep him from being a total blank slate character.

Speaking of which, the MC of Persona 3 is someone I also grew really attached to, which is what made playing through The Answer so powerful because I was kind of in the same emotional state as the rest of the cast concerning the ending of P3. Elizabeth was also a character I really grew attached to thanks to the addition of her "dates" side mission. The last date in particular kind of blew my mind tbh... Honestly, in terms of writing good characters, I feel Persona has found a really good means of immersing the player and getting them attached to the characters. It's difficult not to form a bond when the game kind of pseudo-similates every day life with them.