MY OPIONION : it is so obvious Mr spirit is the main lead but i kind of want to see it happen. since in some sense he did raise her but didn't. Mr spirit only came during the winter and taught her the basics of life and about her powers kind of having a mentor role in her life, and she only saw and referred to him as an friend. Mr spirit isn't human so you do not know his age, he's a demi god spirit like being so i guess it makes this trope a bit more bearable
i personally didn't see the romance coming but i think it was well played. since she has to live longer than all her family and friends having the main lead be immortal and some she shares a deep bond with seems logical
i can see why people are disgusted since he did help raise her but overall i think his involvement is just a well played move and i think this manhwa actually did this trope well compared to others which just purely seems like borderline grooming.
And to finish of this trope overall is just borderline grooming and it is a blessing that doesn't get used a lot