Most of the song is pretty easy to understand; The monster comes along to the girl's home aiming to consume her life force, but she isn't having any of it. She stops them and convinces them to try to eat some regular food with her. Over time they become close; Close enough to consider each other family. From the girl's claims of being a "mad scientist" and that the monster is "[her] only retort" it's likely she's a loner without any real family or friends before this monster came along.
But what gets me the most is the imagery I get from the last chorus. From the picture we know they were sitting down for dinner as usual when this figure, likely from the government, comes in and shoots the monster, killing them. The lyrics say their head BURSTS, which means blood and pieces of flesh would splatter everywhere; On the table, the walls, the food, and the girl herself who's sitting right across from them.
As if that wasn't enough, the girl then proceeds to collect all the pieces and slowly puts the monster back together piece by piece and replacing what couldn't be salvaged. From the lyrics we can tell that deep down she knows that her friend is gone and never coming back, but she is unable to cope with this and seems to begin to delude herself. She continues to list off foods for them to eat and states that she's still seeking for their "RTRT/Retort"
From these pieces of information it seems likely that the girl took the monster's reconstructed dead body and continued to sit them at the table, serving food every night and speaking to them as if they were still alive waiting for them to actually reply. Pretty sad and dark