Seijo no, Isan

This is the first entry in the "Saint" series by Mutsuhana.

Nae Katamine, a college student who became completely alone after the death of her grandmother—her only living relative—discovers a portal to a sanctuary in another world through an old wardrobe left behind in her grandmother's belongings.

There, she meets a boy covered in wounds named Sacred. Unable to leave him behind, Nae begins to travel back and forth between the two worlds. Over time, as Sacred grows taller and more mature with every visit, their relationship begins to transform into something more.