Katoa is mentioned by his father, Basia, then again by Prax in the tenth episode of the second season, "Cascade"
An image appears in the early season teaser trailer that represents the character having been hybridized by protomolecule, a process that was not completed in the novel.
The same image appears on the monitors in episode 3 and episode 4 despite the character himself not having achieved that state at that point.
A later S3 trailer shows Mei in an encounter with another child, presumably Katoa, in a moderate stage in the hybrid transformation process. Those scenes appear in season 3 episode 3.
The character in the early and intermediate stages of protomolecule infection is portrayed by Jaeden Noel. The character in advanced stage of infection is credited as Katoa Hybrid and is portrayed by Jordan Dacol.