Sejuani is the only champion in the game that does not have a standard magic resistance scaling (30-38.5 or 32.1-53.35), though she does share this trait with Mega Gnar as he has different base stats to Gnar
Sejuani leads the Winter's Claw tribe, currently engaged in a civil war against the Avarosan (led by Ashe) and the Frostguard (led by Lissandra) for the Freljordian throne (she believes only the strong deserve to survive)
Sejuani has managed to recruit Olaf (who wants to die gloriously in battle and joined after fighting her to a standstill) Udyr (whom the spirits told him Lissandra's corruption is rapidly spreading across the Freljord) and Volibear (who believes the Ursine can only defeat Lissandra's forces with the Winter's Claw's help)
The circumstances of how Sejuani stole the Seeker's Armguard from Lissandra are currently unknown (likely scenario involves a raid on Frostguard territory)
Sejuani descends from Serylda (one of the Three Iceborn Sisters) making her Ashe's's (Avarosa's descendant) distant relative and Lissandra's's great-niece-many-times-removed.