Lord Sanguinar aids Kael'thas along with three advisors during the boss encounter in the Eye. He appears to be a Paladin. Kael'thas refers to him by the title "The Blood Hammer".
Speculation
His name suggests something to do with blood or bleeding, because of the "Sanguine" reference and that the suffix -ar in Spanish verbs is the infinitive.
He and Valeera Sanguinar are likely to be related.