Pull content from both tablers

If I understand correctly, this should be what you want:

var usersKommentar = from KommentarA in db.forumkommentars
                     join BrugereA in db.brugeres on KommentarA.brugerid equals BrugereA.Id
                     where KommentarA.Id == getid
                     orderby KommentarA.Id descending
                     select new { // change/add property names below as required:
                                  UserName = BrugereA.UserName,
                                  OtherUserInfo = BrugereA.Something,
                                  Kommentar = KommentarA.Kommentar,
                                  Date = KommentarA.Date
                                };
 
RepeaterKommentar.DataSource = usersKommentar;
RepeaterKommentar.DataBind();

We're returning an anonymous (compiler generated) type here in order to get columns from both Users and Comments. As I don't know how your columns/properties are named, I just wrote some examples there. Please change and add as you need it.

One thing looks to me like it's probably an oversight from you: You're ordering by KommentarA.Id and also filtering by KommentarA.Id. One of those lines doesn't make much sense or maybe you intended to use some other column for filtering or ordering.

Edit after comments:

An alternative way, if you don't get the above to work with the Repeater:

class UserKommentar
{
    // change/add properties as required:
    public string UserName { get; private set; }
    public string Kommentar { get; private set; }

    public UserKommentar(string userName, string kommentar)
    {
        UserName = userName;
        Kommentar = kommentar;
    }
}

// and then:

var usersKommentar = from KommentarA in db.forumkommentars
                     join BrugereA in db.brugeres on KommentarA.brugerid equals BrugereA.Id
                     where KommentarA.Id == getid
                     orderby KommentarA.Id descending
                     select new UserKommentar(BrugereA.UserName, KommentarA.Kommentar);
 
RepeaterKommentar.DataSource = usersKommentar;
RepeaterKommentar.DataBind();