Relax, I'm a Ninja Series by Natalie Whipple

A Clan of ninjas in San Francisco may sound improbable-but as the son of a ninja Master, Tosh Ito knows what lurks in the shadows of his city. Or at least he thought he did. When a killer with a poisoned blade starts cutting down teens, Tosh enlists Amy Sato-newest ninja recruit, and his best friend's crush-and sets out to uncover the killer's identity. What they find is ninjutsu more evil than they could have ever imagined... moreA Clan of ninjas in San Francisco may sound improbable-but as the son of a ninja Master, Tosh Ito knows what lurks in the shadows of his city. Or at least he thought he did. When a killer with a poisoned blade starts cutting down teens, Tosh enlists Amy Sato-newest ninja recruit, and his best friend's crush-and sets out to uncover the killer's identity. What they find is ninjutsu more evil than they could have ever imagined. As Amy and Tosh grow closer, they discover their connection unleashes a legendary power that could stop the murders. Problem is, that power may be exactly what the killer is looking for, and wielding it could cost them their souls. less

Sometimes it sucks being a ninja. Still reeling from their disastrous face-off with the demon Akuma, Tosh and Amy are desperate to get stronger. They gained a small victory last time, but the price was too high, and it keeps getting higher. Akuma isn't taking any chances. While he hunts the Inyo's ancestors in the kami realm, his ninja clan targets their living relatives. Each soul consumed weakens the Inyo more, and one devastating blow threatens to take them out of the fight completely... moreSometimes it sucks being a ninja. Still reeling from their disastrous face-off with the demon Akuma, Tosh and Amy are desperate to get stronger. They gained a small victory last time, but the price was too high, and it keeps getting higher. Akuma isn't taking any chances. While he hunts the Inyo's ancestors in the kami realm, his ninja clan targets their living relatives. Each soul consumed weakens the Inyo more, and one devastating blow threatens to take them out of the fight completely. With family dying in both realms, Tosh and Amy are running out of options. Unless they can discover a new source of strength, they will be forced to do the unthinkable: Sever their connection to buy the living realm some time, or lose their souls-and everyone else's-to Akuma. less