Harry potter's twin Poseidon and amphrite's daughter adopted issei fem oc
Harry isn't a demi god the godly eccense could only produce one child Lily and James thought they couldn't have children cause James was hit with a curse they briefly went to america during a break in the war meet a ritualist at a curse specialist they gave a ritual to invite/invoke a god to bear children Lily was unknowingly pregnant with Harry even the gods didn't notice because it was so early.
Alyssia is the twice prophecy child aka Percy and Harry
Dursley's abandoned her in Japan at a Ameritsu temple as a baby Goderu Issei's dad is a friend of the head Priest who knew of the troubles to conceive so called him to come adopt this abandoned but healthy baby girl though not before informing both the head of the Ministry of Magic and Yasaka who was already informed of the demi-witch by Ameritsu and have them check on her by doing a blood test
Goderu and Mikki must be informed of thier new childs states and Yasaka and the Ministry must provide her with tutors for the Supernatural because i can't see the new parents not arguing for her to have a semi-normal life especially for what she will eventually go through (i could totally see them trying to fight the Satans in cannon for Issei because he was their son after so many miscarriages no way a normal parent would put up with Issei's behavior other wise pretty sure the miscarriages are cannon or i could be confusing it with fanfic). Alyssia has to join Rias but only after they had been friends for awhile she has inherited the fatal flaw of loyalty and the saving people thing though it only applies to those she is loyal to or who are loyal to them(ex being loyal to Artemis but not Apollo or Zoe but would save them if Artemis asked or was going herself). For the Triwizard tournament either it dose not work with her name or it happens latter but it must be while she is in Rias's peerage and while she is involved with the percy jackson verse mainly for thier reactions