u/the_fake_ghost

Just imagine subaru just teleports onto the world of undertale falls down the mountain into a bed of flowers and while thinking another person falls down and he catches them(frisk) a cute child and goes into the underground to escape

At the start of the story frisk is quite like in cannon but starts to bond with subaru so much so they start having small conversations.frisk only talks to subaru

To everyone else she only says one sentence,refining

Their relationship.counting on Subaru to make conversation with other monsters and frisk would sometimes recommend action like use her stick to play fetch with the dog monster

Frisk & subaru during the torial fight torial accidentally kills frisk As subaru watches and then frisk drops dead chard and then 15 seconds later subaru respawns but he didn't die but when he looks at frisk , subaru see's confusion and fear on frisk face and then it clicks on him what happened and as the story progresses subaru helps frisk with her abilities and at the start of the story subaru gains an ability which is healing items

Frisk abilities:

Frisk has superhuman stats but only by a little bit like for example she destroys the dummy with one swing

Of their stick (please don't do it in the fanfic i want the pacifist run of them i just wanted to show you an example) and is very agile and is fast like 20kph but frisk is not very durable(for the sake of the fanfic) and has soft cheeks and hair and very adorable (which is my headcanon of why the undertale monsters do less damage then their cannon atk stat which is backup by the item the faded ribbon description "if you look cuter the monsters might hit less hard")

Subaru gained abilities:

Healing items: can you use healing just like frisk

And all stats are the same as in re zero but is more durable than frisk

Frisk and subaru would share a deep connection with one another because of frisk's experiences with death that only subaru can understand and Subaru hides why the fact that he knows the feeling of death and frisk being a child wouldn't pick up on it and because of the strong bond frisk would open up about their life and why they jumped off into the hole in the mountain (this would be perfectly set on waterfall because of the atmosphere and the perfect amount of time their bond to be set in)

And the rest I will leave it in the hands of the fanfic writers