Momiji and Rachel are guest characters definitely but the more I think about Hayabusa, the more I realize he's not exactly a guest character. Reboot Hayabusa's first appearance wasn't in Ninja Gaiden 1 (Xbox) but DOA1 and he also appeared in DOA2 (having a big role in the story of DOA games overall might I add) before appearing in the reboot of NG.
Free Step Dodge
ChaolanLegacy
Onryoki
To me he’s like Yoshimitsu, they’re both originals yet guests at the same time. Because he appeared in the very first DOA and NG. Momiji and Rachel however are not. Rachel and Momiji got added 9 and 5 years after their debut in ninja Gaiden. Even in their trailers they show the ninja gaiden logo making it very clear they’re from ninja gaiden and that they are guest characters.
Onryoki
They could become DOA characters if they are added to the base game as main characters instead of as DLC characters.