She wants to marry him to thank him for saving her Ep4

Plot : Yuya Tenjo is an overweight, short and meek middle school student who is bullied at school and tormented by his family, except for his grandfather who is the only person who cares for him. When his grandfather dies and leaves Yuya as the sole inheritor of his house, his parents expel him from home, and he is forced to live there alone. Inside the house, Yuya discovers a portal to another world, where he obtains the power to change his life forever.

anime : Level Up wa Jinsei wo Kaeta (EP 4)