Warning: Her involvement is this route is mostly meant as a possible secret happy ending I came up with. Most may consider it Mary-Sue so read at your own risk.
There are two ways for Umbrieal to be involved in the genocide route. One where she is "spared" and one where she is not. If she is spared it means the player didn't go to her house to kill her and her father. As such the both of them fled to Hotland to hide.
If the player "does" go to her house, a fight between them and Umbrieal will start up. Like in pacifist and neutral, Umbrieal will only use defensive magic, even when the player is trying to kill her. However, unlike every other monster, she will flee once her HP gets incredibly low. Where she goes is unknown but it is likely she is following the others and left to go heal. She can be found at her house either before or after the player kills Papyrus but, if they enter Waterfall and then go back, she'll be gone.
If the player goes through with genocide and fights Sans, Umbrieal will appear before Sans can be killed by pulling him out of the way with her magic chains. She will then offer the player a chance of a happy ending and take over the fight, ordering Sans to regain his strength. Unlike earlier, Umbrieal will fight the player for real, though will "try" not to kill them but disarm them and contain them instead, while lowering their HP.
As she fights the player, Umbrieal will constantly hit them with bursts of yellow light, which reveals to be the source of her magic; forgiveness. By doing this, she hopes she can make the player see what they are doing is evil (guilt them in a sense) and have them do a reset. She will also reveal that she can vaguely remember other timelines through her dreams, having once thought they were only that. She comments it may be the combination of her human and monster soul that allows this, but she's not sure.
If the player dies by one of Umbrieal's attacks, or by Sans who will join the fight if given enough time to rest, they will be given the choice of continuing or reseting. If they continue it will be just Sans in the judgment hall and he will comment on their heartlessness of turning down the second and only chance of redemption they got. This implies Umbrieal may not come to save Sans the time around.
If Umbrieal dies, then the player will have one last fight with Sans, who will still be tired out from his last fight. If the player dies, the same thing will happen if the player refuses Umbrieal's gift of redemption. Afterwards the genocide route goes on as normal. She, like everyone else, will die when the world is erased and will have no memory of the player once the world is revived.
Umbrieal will only intervene in Sans' fight if the player has tried to kill her or has killed her father and boyfriend. If she went ignored, or her boyfriend and father went unharmed, she won't get involved and the genocide route will go on as normal. Not killing the boyfriend and father won't effect genocide route. The player just needs to avoid going to Umbrieal's house and avoid going to the boyfriend, as he does not effect the count/kill meter.
The bursts of yellow light is actually inspired by the Glitchtale animated movie. Umbrieal, despite not being a cowered, can flee a fight do to her human logic of "if I don't move, I'll die" like Sans' logic of dodging. At least that's the best I can come up with
