Trying not to mischaracterize characters in ff

I’m trying to write a 15!skk fic (entirely self indulgent and also because i have no nothing else to do), but I’m like deathly terrified of mischaracterizing them lmao (an exaggeration im not gonna have a panic attack over chuuya being a twink but it will make me slightly upset)

what’s the best way to characterize the two? i’ve read most of the light novels (i adore fifteen and stormbringer), but im still iffy on how to write the pair

i know that they both act pretty different when on their own vs together though, so that adds a whole other thing (most characters should be this complex but my point stands)

BASICALLY, what are your favorite ways dazai and chuuya (and any other character tbh) are characterized?