Lancelot

Knight of the Lake
Raised by the Fairy of a lake, his sighting of King Arthur made him fall in love with the idea of becoming a knight, and due to his latent talent went on to become one of The Knights of the Round Table, and not only one of Arthur's most loyal knights, but the very image of the perfect knight in Arthur's eyes. However, knowing the burden that his king carried, he tried to talk with dear his wife, Guinevere, and that's where he learned the secret—that his king was actually a woman. The King's wife had married just to preserve the image of a normal royal family, and Sir Lancelot couldn't help but be feel pity for her, which would turn into attraction. When political enemies exposed the adultery, she was sent to be executed, and even though he went as far as to kill 3 of his comrades, he couldn't stop her death. This resulted in the fracture of the Round Table's Knights, and ultimately civil war that destroyed the country. When Lancelot heard of the death of King Arthur, the king he once he so fiercely admired and vowed loyalty, caused by his own actions, he descended into madness, tainting his very Spirit.

Fate/Zero
Summoned by Kariya Matou, he was brought as Berserker, a Servant class that sacrificed it's mind for pure power. As Kariya is a weak Magus, and Berserker few thinking capabilities seem to somewhat remember Saber, he tries to savagely attack her at every opportunity regardless of his Master's command.

When attacking Caster, Berserker takes control of an F-15 Jet to use as his Noble Phantasm, but is distracted when he sights Saber, allowing him to be defeated.

On the final of the 4th Holy Grail War, Berserker engages Saber, but as he is exceptionally good at battling her, Saber deduces he must have been related to her and breaks his helmet, revealing the maddened Lancelot, which destroys Saber's will to fight as she is overcome with guilt. As Berserker brutalizes Saber, he stops before the final blow, his mindless attacking having fully drained his Master of magical energy, and proceeds to fall and regain his mind as his class skills can no longer be sustained, and exchanges a few words with Saber before dematerializing.

Season 3
Someone had summoned Berserker again to battle Archer, and using his overwhelming strength he managed to win, as Archer's abilities seemed to conflict with The Laws Of Wrestling. Who could have summoned him...?