Eventually, Blue and Lucario had gotten into a fight over the fact that she was unable to protect Lua from Calibri. After Blue had gone home for the night, Calibri had broken into the UMH and had killed Lucario in his sleep, transferred his dust into an hourglass that contained the word "LUCARIO" scratched into the wood at the top and bottom. The hourglass was then left it to the side of his bed.
Lucario is an undead Lucario Skeleton monster with a green straight jacket (that he requested) that he must wear at all times. He also has a pair of green earmuffs used to silence the screams of UMH during the night. His eye sockets have small green pupils with a white center. And, finally, he has a tail with 3 segments, but the other 2 have been taken, so he could not use them to attack.
Blue (Early in the MentalTale timeline)
Lucario is in a one sided "relationship" with Blue, a nurse he has become obsessed with. Blue dose not seem to have any feelings back, acting very distant and cynical towards him.
Blue (Later in the MentalTale timeline)
Blue eventually gives in to Lucarios feelings and decides to give him a chance. During this part in the timeline, he is allowed to have his straight jacket off and even walks freely through UMH from time to time. (But only if Blue is there to keep an eye on him)
Lucario and Papyrus are friends in UMH, Papyrus seems to get timid around Lucario as he does most people, however.
Lucario and Sans seem to have some sort of beef with one another. Both of them act very cynical towards one another and try to get the other into uncomfortable (and sometimes dangerous) situations.
MT!Gaster (Before "Bluca" becomes canon)
Gaster does not trust Lucario around Blue and will summon a wall of Blue Bones around her if Lucario gets close to her. He also seems to dislike Lucario being near Papyrus and Sans, but especially Papyrus.
MT!Gaster (After "Bluca" becomes canon)
Gaster still does not seem to trust Lucario. When ever Lucario and Blue are in the same room with him he always seems to keep an eye on Lucario. When He and Blue are alone, he constantly gives Blue warnings about Lucario and warns her that she is making a big mistake.
Lucario respects Toriel and tries to act as respectful (and sane) as he can whenever she is around.
Lucario respects Asgore as well but seems to not enjoy his company, As Lucario believes that Asgore does not care about the residents of UMH.
Lucario and Asriel have hardly seen one another in UMH. They have glanced at one another when they are walking in he same hallway, but other than that, they have had no interaction.
Lucario does not fully understand Flowey as he has not been around Papyrus long enough to understand to severity of it, but he does believe Flowey is a real entity and tries to calm Papyrus when he comes to torment him.
Lucario does not like MK what so ever. He and him are getting into constant (sometimes deadly) fights. One of the biggest reasons for these fights being so deadly is that they are both restrained by straight jackets and know they could destroy the other if they could break out of them.
Lucario and Alphys are very good friends in UMH. They met during one of Alphys rehabilitation routines. They both enjoy anime and they talk alot, the only thing that they seem to not like about one another are there differing opinions on Mew Mew Kissie Cutie 2.
Lucario seems to really enjoy Mettatons company and is a huge fan of his show. It seems the conversations he has with Mettaton are some of his most sane.
Lucario does not mind Undynes company but is always timid as to not anger her. He isent really the best with keeping up with her violent tendencies.