Quick answer — Rin Kurusu performed 1 anime theme, including "Believer" from That Time I Got Reincarnated as a Slime Season 3.
Rin Kurusu performed the anime theme songs "Believer" (That Time I Got Reincarnated as a Slime Season 3).
Rin Kurusu's first listed anime theme is "Believer", the ending of That Time I Got Reincarnated as a Slime Season 3.