Fate/Grand Order - Daily Chaldea (Doujinshi) - Ch. 681 - Summoning Systems

[MangaDex]

 

[Hikari_Owari]

If it works, it works.

 

[OmegaWanderer]

Astolfo, you're not too good of a Paladin either

Don't worry tho you're still best boy.

 

[Krune]

its magic it just works I believe someone said once... possibly regarding a divorce/annulment

 

[SonicDisgaea98]

Technically, Ritsuka doesn't summon them. It's Shadow Servants, just replicas of them that lack the actual autonomy. The reason Ritsuka can do it is unknown. It's probably just some unique ability they have like Emiyas reality Marble. Something to make them stand out and be a worthy protagonist.