Fate/Apocrypha x Highschool DxD
In which Issei Transmigrated into Sieg's body after he is poisoned by Samael.
Dying by Samael Poison, Great Red and Ophis decided to save Issei, but instead of creating a new body for him they just went and find some random host for him. Escaping from Yggdemilia, while carrying Ophis by his side, and then meeting the Servants during their fight.
"Oppai!" Issei
[Is that seriously your first thought seeing warriors fight their battle.] Ddraig
"What are you talking about Ddraig I'm talking about that Blonde girl, she has Oppai as big as Buchou!" Issei
[...Please do think, about how you look now. You wasting this new body looks.]Ddraig
"Ddraig, I'm sleepy. Let's rest before going home." Ophis