In the previous paragraph, Rambam Hilchot Teshuvah 2:9, he writes (Sefaria translation):
[...] but sins between man and man, for instance, one injures his neighbor, or curses his neighbor or plunders him, or offends him in like matters, is ever not absolved unless he makes restitution of what he owes and begs the forgiveness of his neighbor. And, although he make restitution of the monetary debt, he is obliged to pacify him and to beg his forgiveness. Even he offended not his neighbor in aught save in words, he is obliged to appease him and implore him till he be forgiven by him.
The one who transgressed against another person is not forgiven by God without first making things right with the other person. There are many passages on Yoma 87a that say the same. If the person never asks for forgiveness and never makes things right, it seems that you have not been called upon to forgive -- the process describes forgiveness as a response, not as something you ever must initiate.
That said, the Yom Kippur liturgy contains this passage (quoting from here):
I hereby absolutely forgive anyone who has harmed me, other than money I can still claim by law, or those who harm me figuring that I'll forgive them. Other than those, I completely forgive, and may no person be punished because of me.
This seems to me to be a request of God, who per the Rambam and Yoma will not forgive people who have not done teshuva. It excludes people who rely on this clause, but of course we generally do not know others' intentions. (Even if they declare them, can we rely on what they said?) I have been told that this "release clause" might not actually be effective, and was pointed to a teshuva by R' Binyamin Zilber, but I can't read it. I asked my rabbi about this once and he understood this to be a catch-all for unknown transgressions -- a person who doesn't know he offended you won't ask for forgiveness, but if the two of you sorted it out, you don't want God to hold this person accountable. I have no source for this interpretation.
According to the Rambam, you can even refuse to forgive when asked (twice), so clearly according to him you can refuse to forgive if you weren't even asked. I've seen a lot of writings about how it is meritorious to forgive anyway, and there's that passage in the machzor, but my understanding of all this is that you're not required to if the person doesn't even make amends and ask for forgiveness.