A term that comes to mind is nice guy. In most contexts, it simply means a guy who is literally nice. It is something both males and females would most often use to praise someone — It's great to be working with him; he's such a nice guy.
"Nice" can have negative connotations, however, indicating someone who is not assertive or ruthless enough, and who therefore will not succeed, or who might be gullible or easily exploited. This gives rise to expressions like no more Mr. Nice Guy and nice guys finish last.
In pop culture discussion of relationships, a "nice guy" is archetypically a man who is unable to articulate his romantic interest in a woman (especially from insecurity about his attractiveness), and who overcompensates by being overtly generous and accommodating, in the hopes that the subject of his attention will notice, return the interest, and take the initiative in advancing a relationship. This is not how adult human relationships actually work, of course, but that does not stop the nice guy from constantly making gestures.
The English title of the 2009 Spanish film Pagafantas is Friend Zone, which may also be a helpful related concept to know.
In pop culture, the friend zone is an imaginary space in which someone is stuck when he or she has a romantic interest in someone, and that second person does not reciprocate that interest—but that second person is willing to maintain a non-romantic friendship. If the second person wanted to have no contact, the first person would be forced to move on. In the friend zone, however, the first person maintains the non-romantic relationship in the desperate hope the situation will change.
Nice guy and friend zone were relatively lighthearted phrases until relatively recently. Now, as with so much else, they have been weaponized by extremists on both the right and the left, so if you are writing anything going on the Internet, be prepared for a torrent of abuse.