Could the reason yasui bonded with zoro was because he noticed zoro's resemblance to shimotsuki ushimaru.
We know that yasui is a good judge of character, he was able to identify the scabbards and shinobu on sight.
If kawamatsu and hyogoro could notice the similarity, yasui should be the first one to do it.
But it's still weird that onimaru/gyoukimaru didn't feel anything during his encounters with zoro.
Zoro might be the last living heir of the shimotsuki clan.
Fk o-toko.