When Yu Yu Hakusho came out, both the manga (1990-1994) and the anime (1992-1994) were universally acknowledged to be groundbreaking works of art. In fact, although the anime only ran for 112 episodes, the series has been consistently ranked to be among the best anime ever created. The main characters are all fleshed-out and complex, none of them being two-dimensional placeholders (a feature some anime adds to a few of their characters.)
Two of them — Yusuke's teacher, Genkai, and (Younger) Toguro, the main antagonist of the Dark Tournament Saga — were revealed to have had an intimate relationship with each other in the past. However, a horrible event in Toguro's life diverted their paths away from each other, leading to opposing outcomes. The given list offers arguments on behalf of the choices of both characters, explaining why they did what they had to do.
10 Genkai Didn’t Sell Her Soul
Fifty years ago, Toguro and Genkai were once lovers, who ran a dojo together for a number of promising students. However, their idyllic life was cut short by Kairen, a horrible demon who murdered all their young scholars, in order to "convince" the pair to enter the Dark Tournament.
Genkai refused, mainly because she could see through Kairen's ploy, and begged Toguro not to go seeking revenge. Her pleas fell on deaf ears, but she still participated in the tournament for his sake. When Toguro chose to become a demon, Genkai was disgusted — breaking all ties with him for good.
9 Toguro Got Unlimited Power
As a reward for winning the Dark Tournament, Toguro was allowed to shed his human form, gaining far more powerful demon energy (although it was only at Class-B.) With his new abilities, Toguro and his older, and far more insidious, brother, went around looking for other strong people.
In fact, one of his earlier opponents happened to be Tanaka, who lost miserably to Toguro in a private battle. He appears in the current tournament as Suzuki, in order to avenge his humiliation.
8 Genkai Sacrificed Her Powers For Yusuke
Given Toguro's years of training, plus his demon energy, Yusuke was at an amateur level at best. To help improve his chances, his master, Genkai, takes him into a cave on the island, where she asks if he is willing to do whatever it takes to win the Dark Tournament. Yusuke has never been one to refuse a challenge, agreeing instantly.
The absorption of Genkai's Spirit Wave technique almost killed her star pupil, but it worked, and Yusuke was strong enough to face Toguro on even ground. This sacrifice, however, depleted Genkai of most of her spirit force, a prime consequence being her death at the hands of her ex-lover.
7 Toguro Did It For His Murdered Students
Toguro was indeed corrupted by his rage, seeking blind vengeance against the demon who had caused him such anguish. Genkai believed, at first, that he had forgotten where his loyalties lay, that he sought power only for himself. However, several instances in the series make a different argument.
Toguro chose to become a demon, because for some warped reason, he wanted to die at the hands of a human, the same way he had killed Kairen. Further, when Genkai tells Toguro (in the afterlife) that she considered Yusuke like her own son, he said that he cared about the kid too, asking her to make sure Yusuke doesn't follow him into hell.
6 Genkai Retained Her Youth Through Spiritual Power
Genkai's usual form was that of an old woman with messy dull-pink hair and considerable facial wrinkling. However, during the Dark Tournament, she takes on a much younger form, at least until Yusuke takes her powers.
When Shishiwakamaru tries to have her blacklisted, Toguro steps up and vouches for her identity — he says that Genkai's Spirit Wave ability has a special utility, aside from attacking and healing, in that it can make a person's appearance resemble what they looked like in their prime (over fifty years ago, in this case.)
5 Toguro Retained His Youth By Becoming A Demon
Instead of toiling over techniques and moves, Toguro chose a much simpler path to the fountain of life. Demons not only have far greater power, but also age far slower than humans (Raizen, Yusuke's ancestral father, starved for a thousand years without dying.)
When Toguro became a demon, his body became resilient to most forms of damage, as seen when Genkai rips his shoulder out in their last fight. He says that "to keep me young, the committee gave me a body that was really quick to heal."
4 Genkai Had People Who Truly Loved Her
Genkai understood the real meaning of life — that it had to be lived to the fullest extent and not stoppered by boons and magic. She becomes the mother Yusuke almost never had, forcing him to surmount his limits with rigid discipline and harsh punishments.
Nevertheless, Yusuke loves her dearly, so much so that a large fraction of his motivation to kill Toguro arises from her death. The rest of the team also care about Genkai, as shown when they run to her (even Hiei) when she returns from the dead.
3 Toguro Preferred Respect To Affection
Although Toguro sacrificed both his humanity and his capacity for love long before he became a demon, he cannot tolerate losing his honor. For instance, when Elder Toguro suggests that they cheat in the final match, Toguro punches him straight through the hotel room wall and into the ocean.
Sometimes, though, his need for respect results in other people suffering, like when he defeats Bui before the tournament, forcing him to kneel and express his eternal allegiance.
2 Genkai Was Not Afraid Of Death
The plucky old lady went to battle with her ex-lover, knowing full well that she stood no chance against Toguro's demon powers. To be fair, she could have put up a real fight against him if she hadn't donated most of her spirit force; as she expects, Genkai is murdered by Toguro.
Considering her total acceptance of the circle of life, she is the only character able to temporarily manifest after her death (in the form of Puu) as Yama and Koenma know that she will return to Heaven when her job is complete.
1 Toguro Accepted His Punishment
On an unconscious level, Toguro was fully aware of his evil deeds, but his demon nature held him back from redeeming himself. After he dies, he goes to Koenma, who proposes a less harsh punishment in view of his earlier good deeds. However, Toguro rejects this offer, saying that he deserves to suffer for 10,000 cycles of 10,000 years in the lowest pits of Hell.
Genkai's spirit tries to reason him out of this decision, but he simply says that his only regret is that he didn't get to spend his life with her, before he walks through the final gates and is lost forever.