sia

Sia is the possessive that points back at the subject of its own clause, so li prenis sian manon is his own hand while li prenis lian manon is somebody else's. It exists only in the third person, because mia and via are already unambiguous. Like every possessive it agrees with the thing possessed, not the owner: sian manon, siajn manojn. It can never appear in the subject, since a subject cannot refer back to itself.