Sumarbrander
Wikipedia
A Guide to Magnus Chase and the Gods of Asgard series by Rick Riordan
The Sword of Summer (Jack) was Frey's weapon, currently wielded by Magnus Chase. After Magnus finds out that the sword is sentient, Sumarbrander decides to name itself Jack, and be referred to as a male. In battle Magnus can let the sword attack his enemies on its own or use his own strength and control the sword himself. Either way, it is Magnus who loses energy ultimately, though this loss is delayed until he next grips or sheathes Jack.
Jack bears a grudge against Frey, who abandoned him, by giving him to Skirnir, as a price for Skirnir bringing Frey a Giantess with whom he fell in love with; he summaries this by once admonishing Frey, saying 'blades before babes'.
He reached Boston with one of Skirnir's descendants during the Viking expansion, where he was lost for centuries till he was recovered by Magnus.
Jack bears a grudge against Frey, who abandoned him, by giving him to Skirnir, as a price for Skirnir bringing Frey a Giantess with whom he fell in love with; he summaries this by once admonishing Frey, saying 'blades before babes'.
He reached Boston with one of Skirnir's descendants during the Viking expansion, where he was lost for centuries till he was recovered by Magnus.