Why is Chen not "Yakumo Chen"?

Ran, explain it to her.

...yes. Listen up, ok, Chen?

You can't take the surname Yakumo until you go through a strict training regime.

But whether Chen would be able to endure the training...

I don't really get it but I want to be the same as Ran-sama!

Got it! Then your training starts from tomorrow!

*chop chop*