Kenma's relationship with Yaku is simply that of a senior and junior. Yaku enjoys talking about the growth of their juniors, and Kenma undoubtedly belongs in that category.

There's even this one scene, where Yaku was emotional when he saw Kenma naturally talking with Hinata and Yaku. He just seemed like a proud senior or actually it was more like a proud mother when his son finally acquired friends.

With Kai, since he have this calm, collected personality, his approach towards Kenma is relaxed as he is a very peaceful person and bc of that Kenma reciprocates Kai's actions by being polite. Just like whenever Kenma thinks his tosses weren't good, he'll apologize to Kai.

Personally, I think Kenma has a soft side for Fukunaga. It probably started when Fukunaga stopped a fight between Kenma and Yamamoto by throwing a bucket of water over them to prevent the situation from escalating. Fukunaga is fond of comedy and often spouts funny rhyming terms.

And there's this one time, he complimented Kenma's toss and who knows whether Kenma smiled at the compliment or bc of the playful term Fukunaga used. He can also frankly tell Kenma whatever he thinks despite the rest of the team acknowldging Kenma as "terrifying."