CallmeMISTER
It is blasphemy that no one knows Cid kagenou, who is the eminence among the som and the only one who behaves like a mob and then reveals his true power. Practically the guy pretends to be a mob but at the same time to be one of the literally most broken characters in terms of magical power, etc. By the way the novel is kage no jitsuryokusha ni naritakute, it has only 4 volumes and it is rumored that this year in October at the same time as the anime comes out, the 5th volume (light novel) will come out.
To be honest, I can see the MC telling he could try and help during the Rating game of Rias vs Riser. Revealing himself when she is going to lose after copying the healing property of the Phoenix Tear in one of his body enhancing spell. While studying unercover, he could also try to "upgrade" his analyse of sword to do something like shirou (learn the technique of the one that used the sword he got), then try to find "good" sword across the world xD.
definitely go for broke and get both why not after Phoenix shows up it's a definite go to grab his sister not only for a love interest but to have your own stock of Phoenix tears yeah I'd say get her he's going to need cash and a lot of it for materials and weapons for later on đ€maybe open a sweets cafĂ© too
I couldn't help but feel that this Aizen arcs will serve as a huge power boost to him. Once he's done analyzing stuff like the Power of Destruction or other traits that the characters has, he'll become strong enough to fight against other (strong) boosted gear wielders. Who knows, maybe he can copy their effects (to a certain extent) himself?