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."

Yamamoto was probably the only one whom Kenma had a hard time getting along with at first. Seeing they often squabble and fight. But later on, it was revealed that Yamamoto actually admires Kenma's determination or what he calls as "willpower" to finish every tasks.

And bc of that, their friendship surprisingly developed along with Fukunaga as they're all in the same year.