Board Thread:Questions and Answers/@comment-2197197-20160911070805/@comment-1210805-20160912151711

A manifested spirit can be seen by anyone and everyone. Renji, Yoruichi and Ichigo could all see both Zangetsu (the old man) and Zabimaru during their Bankai training in chapter 133. Nanao also saw Kyōkotsu in chapter 651. A manifested spirit can be seen by anyone who can see spirits.

It should also be pointed out that manifesting the spirit is very different to actually achieving Bankai. Kenpachi did not know his sword's name until his training with Unohana, therefore he couldn't use his Shikai or his Bankai. If we want to look at things as they went in the story then it goes like this;

Kenpachi finds a little girl among the corpses, she has no name and likes his sword. He names her Yachiru and adopts her.

Yachiru watches Kenpachi's battles, rarely participating but sometimes giving advice.

Kenpachi hears his swords voice and calls its name.

Yachiru uses a "Shikai" that Isane notes doesn't work like any other Shikai that has ever existed.

Yachiru disappears when Kenpachi activates his true Shikai for the first time.

Yachiru reappears, seen by no one but Kenpachi, when she shows him how to use his Bankai.

When Kenpachi's arm explodes, Yachiru notes that she released too much of his power.