User blog comment:Kaido King of the Beasts/Top 10 GOOD Things About the Blood War Arc/@comment-29359011-20160827030619/@comment-493809-20160829074304

The steps to getting Bankai are:

1. Project your inner spirit into the world 2. Commune with it 3. Reach an understanding and convince it to let you draw upon its true power

The only other characters we see learn Bankai are Ichigo, who basically cheated, and Renji, who pulled it off in the hospital. As such, the fact that Yachiru is/was the equivalent to Lilynette isn't all that far-fetched.

And compare the fact that he gave Yachiru her name to how Yumachika(sp?) calls his Zanpakuto the wrong name and thus has a weakened Shikai most of the time. When Yachiru first showed up he had no idea how Shinigami worked (plus he, like Ichigo, didn't actually have a Zanpakuto as neither received an Asauchi), but as a spiritually powerful being he still had an inner spirit. She got projected as a small girl who liked swords and blood (which really should've been a clue), and he gave her a name that mirrored his own.

As for the subjugation, I feel it'd be more accurate to describe it as reaching an understanding with the spirit so they'll let their power flow properly. For Kenpachi, who was basically Yachiru's friend, she had no problem letting him access his Bankai once he remembered how to fight seriously (which unlocked his Shikai) and was in a situation where even more power was needed.

Overall, I felt it made as much sense as anything else in the series (not the arc, but the series). As Khaotic said, he's been doing the equivalent to Bankai training since long before he became a Shinigami.