Alduin - Relationships

Paarthurnax
One of Alduin's younger brother yet an old dragon in our eyes. He served as Alduin's lieutenant during the first Dragon War. He admits that he has committed and seen unspeakable atrocities upon humans. It wasn't until much later that he gathered other dragons and betrayed Alduin as they taught humans the Thu'um. This lead to to human reach a victory and Alduin was banished. Alduin calls Paarthurnax treacherous.

Dragonborn
The only one who can defeat him despite his soul not being able to be absorbed. He has, however, admitted that they have grown stronger and also likes to taunt the Dragonborn.

Mankind
This covers all races in the game. He looks down on them and has enslaved them in the past. Definitely not invited to their parties.