Kai- said:
If Rin had gotten a Saber instead of Archer, Shirou probably would have gotten EMIYA, which would have been a bad end.
The thing is, without the catalyst for Saber, Shirou would probably have not become a master
at all, since he has no real connection to the Grail Wars. Also, if Shirou had summoned Archer, Archer would almost certainly not be able to kill him whilst he still possessed command spells (Caster had to get her master to use up all three command spells before she could).
The reason for that is Arturia most likely can't be an Archer and it is logical to assume that you can be a catalyst for yourself.
Possibly. However, Counter Guardians are not
normally summonable, and I'm not sure that Shirou's body is enough of a catalyst to overcome that.
Although if Arturia could be an Archer, he'd probably just get her (but I don't think she can be).
Definitely, if it was possible (which, like you say, it probably isn't). I do wonder, though, if the Grail somehow "fiddles" things (by pre-emptively allocating servants to classes) to ensure that every master can summon their servant, since there is no situation in
either war where a master was unable to get the servant their catalyst was intended to summon due to there being no free "slots" for them).
If a different class was open as the last spot, it'd probably just be a roll of the dice since Arturia and EMIYA are the only things he has catalysts for.
Or, alternately, the Grail somehow "invents" a new class to put them in, or else just shoe-horns them into whatever class is left.
For instance, Assassin should really be in the
Saber class, since his method of fighting fits that class much more than it does the class "Assassin" (which he's not), and I doubt Caster changed the incantation in order to specifically summon an Assassin (which is what you are
supposed to do). However, putting him as a Saber would have left Shirou with only Assassin to choose, and he has no catalyst for an Assassin, no connection to them (since they're all Lawful Evil, and fight in a way that goes against his fundamental principles) and would not use the special ritual needed to obtain one. So, most likely, the Grail "cheated", and summoned Kojiro (or, rather, the Wraith that was 'impersonating' Kojiro) into that class, since Caster was already breaking the rules
anyway (similarly, when the Einsberns cheated in the third war, they got an entirely new class, which replaced the Berserker class).