Context: characters see a girl appearing out of nowhere and are confused about where she came from. It's the protagonist, who has psychic powers, including the ability to turn into a girl.
touzen, {boku ga nyotaika shita} ni kimatteru darou 当然僕が女体化したに決まってるだろう Obviously it's that {I turned into a girl}. Of course it's me turned into a girl.
don ドン *sound effect used when something comes up suddenly.*
Saiki Kusuo motoi Saiki Kusuko ga na 斉木楠雄もとい斉木楠子がな Saiki Kusuo, or rather, Saiki Kusuko [did].
He, or rather, she is saying who nyotaika'd, since it's marked as subject by the ga が particle.
The name Kusuo ends in a kanji for "male," osu 雄, making it obviously a male name.
The name Kusuko ends in ko 子, which is a common suffix in female names, and typically used when coming up with a quick name for the girl version of someone.
