This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Soul Series
Siegfried's story in general. He killed his own father and his mind broke to the point where he had to convince himself someone else did it just to stay sane. Despite being billed as The Hero and even being named after one, he takes a very tragic and hard fall to darkness. He grabs hold of Soul Edge, only for it to mutate his body and shatter what little sanity he had left, transforming him into Nightmare. Once he's freed from the sword's control, Siegfried is left wracked with guilt over the thousands of innocent people he murdered. Even then, however, his freedom is short-lived, as he still kept Soul Edge around him so that no one could find it and that he could dispose of it later, but he ended up back under the sword's control eventually. He has to be freed again, but by this point he's become a Death Seeker who thinks his sins can never be redeemed, and he pushes away everyone who tries to help him, out of fear they'll get hurt or end up dead. He gets better in V, but VI appears to Retcon his somewhat-happy ending away.
Sophitia's ending in Soulcalibur IV. Despite the initial Narm, Algol's words to her conveys his sadness over being forced to kill his own son.
Algol: "Time is cruel. Even with all of this power, I cannot undo the past..."
Sophitia's whole motive since SCII has also been to protect her very young children. Becomes Harsher in Hindsight in SCV.
Parts of Tira's ending somehow make the psychotic, bloodthirsty, Magnificent Bitch... into The Woobie. It's just so sad and pathetic, as she's begging Nightmare not to leave her alone...
Considering how Tira always sought companionship, it's even sadder. After her time as a member of the Bird of Passage, she was more or less adopted into a family for a short period—but her insanity caused her to Kill Them All when she got into a small squabble with them.
Lizardman's backstory in Soulcalibur III is heartbreaking when you discover that he is a man stuck as a reptilian killing machine who can't speak, forsaken by everyone, even by his god, and desperately (and vainly) trying to get his humanity back while his monster side engulfs him more and more.