k-kuja:

He might have just… never developed one tbh. Ongoing cannibalism (and the massive influx of RC cells that goes with it) can trigger the mutation but it’s not an inevitability; apparently it’s pretty rare. Plus there are ghouls, like the Tsukiyama family, who cannot develop them for genetic reasons – there’s nothing in the text to suggest Yomo or his family share a similar trait but I guess it’s a possibility.

I really like the idea that he can produce a kakuja and chooses not to though – he and Arata were headed down the exact same path, and it took Arata’s death/Touka & Ayato’s abandonment and Yoshimura’s patronage to show Yomo how destructive and pointless his need for revenge was. Rejecting the power he was able to amass in the way of a kakuja would be a nice symbolic nod towards his character development, and I think ensuring he was able to protect his niece and nephew without the destructive power that killed Arata brings with it a sense of atonement for what (Renji feels) he forced their father to do.

It’s also thematically kinda cool considering his advice to Ken: that everything he accomplished was through his own strength and not through the strength he was ‘borrowing’ from others (Rize, Yamori). Forming a kakuja necessarily requires one person to ‘steal’ [kill and consume] from others and, unlike Kaneki (who was a victim of Rize, Kanou, Yamori), Yomo was an aggressor. He probably orphaned kids just like Touka and Ayato.

Choosing not to use a kakuja, if he’s able, could be an acknowledgement that his earlier actions were wrong or selfish; or evil, if we’re going full Kuzen.

He still does become insanely powerful but he’s strategic and incredibly well-rounded rather than being just another big ol’ one punch monster. Even Mutsuki calls him a perfect ghoul, and he defeats his equal even though Uta DOES resort to a kakuja in the end.

When Uta says “you’ve become really strong, Renji” after their final fight, he means since Ren left the 4th Ward – it’s a different type of strength, beyond whatever he was pursuing before Anteiku. He’s become strong in spite of the path he was on in his youth, not because of it.