I can't say that I was too much a fan of Gashapon games, but "Saber", the original from Fate/Stay night was always cute in my eyes. Yeah, I know Arthura Pendragon's face is reused in dozens of Fate/Grand Order heroes, but the original's personality was pretty good. And lothe am I to admit it, the original sex scene (although arbitrary) between Saber / Rin's enchantments + Shiro is kind of hot.
For starts, Saber is a very stoic knight summoned by the main character, Shiro.
A veteran of the 4th Holy Grail War, Saber fought under Shiro's father (although Saber's extremely pissed about it and barely talks about the events). Initially, Saber is distrustful of Emiya Shiro (due to the events of Fate/Zero, she has good reason), but she learns that Shiro has a kind heart and fully accepts him as her master.
Unfortunately, Shiro (the main character) is such a n00b mage that he's unable to sustain enough mana to keep Saber at full power. Should Saber actually have enough mana for sustainment, she's the 2nd most powerful servant in the 5th war (after Berserker: Heracles), and I'd argue #1 most powerful in the 4th war. Saber does need Avalon / the sheath to beat Archer/Zero: Gilgamesh, but she does 1v1 Gilgamesh and beat him in an all-out duel when both characters are at maximum strength.
Full immunity to magic below B-tier, anti-army class Phantasms, regeneration, speed, strength, technique. To top it off, her holy sword Excalibur is invisible... although the skill of the servants are often enough to compensate (Assassin can read the length of the blade based off of Saber's stances, while Lander is quick enough to compensate for the invisible sword), Saber needs to keep her sword secret to keep her identity (and therefore her weaknesses) secret.
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So yeah... Shiro fails at providing mana, and Saber inevitably weakens: to the point where even just walking was a huge strain on her. The main characters Rin + Shiro get chased down by the one and only truly superior foe: Berserker (aka: Heracles). Rin's Servant Archer, was left behind to slow him down. But it was clear that they needed Saber to join the fight ASAP.
Which brings us to the first "mana transfusion sex scene" of all of Fate/Stay series. Rin pins Saber down, enchants her physically (With close contact + Saber's weakened state, Rin was able to enchant Saber through her magic barrier). With Artura Pendragon's normally stoic personality enchanted away by Rin's magic, so she becomes ready for the dicking by Shiro, for the appropriate mana transfusion.
I kind of hate it since it is a super arbitrary sex scene in the middle of an important fight. But... hey, Fate/Stay was originally a hentai, so its all about the sex scenes right?
The story probably works better without the sex scene though. I mean, Archer literally dies while these characters are porking. I understand its to get Saber's mana up so that she's ready to fight but still.
Its not all fun and games. Shiro is at his weakest in all the routes because of this. The mana transfusion permanently weakens Shiro's magic circuits (so it may power-up Saber, it weakens Shiro significantly), so Shiro will never gets to his full potential in this route. (Fate/UBW is probably the cannon route: where Rin "shares her mana" with Shiro, so Shiro is significantly stronger there)
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The Fate/UBW route Saber is of course the "Caster loves to play dress-up" Saber-wedding dress scene. When Saber's (inevitable) weakening starts to kick in, Caster in the UBW route captures Saber. With a scene with S&M implications.
No sex here, but "Caster likes to play Dressup" wedding dress Saber is pretty cute too. Saber plays a far less important route in the UBW route, its Rin's route after all.
. Here's an artwork of Scáthach (from Fate/Grand Order). She's supposed to be some immortal warrior, but with her arms and legs pinned down, she certainly looks like a very much mortal punching bag:
