In Shadow and Bone season 1 initially reveals that the Dregs' plan is to capture Alina for a reward, but Inej lets her go as a gesture of faith.
"The Heart Is an Arrow" changes the entire storyline in Shadow and Bone season 1, and Inej's motivations are teased in the previous episode, "Show Me Who You Are." During a public light display, Alina proves that she is indeed the prophecized Sun Summoner, but doesn't realize that the Dregs are plotting to kidnap her. When Kaz Brekker (Freddy Carter) and company track down Alina, she immediately senses danger and refuses their offer to guide her through the Fold via a secure route. Alina then uses her light powers on the group, and seems entirely perplexed by Inej's ability to weather the storm, along with her display of peace.
Inej's genuine faith in Alina makes her immune to the Shadow and Bone's sixth episode.
Naturally, Alina doesn't fully understand why Inej isn't affected by her light powers. What's important, though, is that she recognizes an act of goodwill. Later in "Show Me Who You Are," Inej defends her actions to Kaz by identifying the Sun Summoner as a "living Saint," or "Sankta Alina" as she calls her after witnessing the aforementioned public light display. The Dregs' mission was fundamentally about capturing a "fake," and the episode's title thematically pinpoints the realities of Inej's existence. She's guided by her belief in Alina as the Sun Summoner. In being unable to hurt Inej, it's a sign that Alina is indeed turning into a living saint and thus her powers can not hurt one of her faithful; Inej's belief is her shield.
The Shadow and Bone season 1 finale, "No Mourners," reveals that Alina is deeply thankful for Inej's loyalty. After weathering a literal storm and fending off The Darkling (Ben Barnes), the women sit by a fire and Inej pledges her loyalty to Alina. "Wherever you are, whatever you need," Inej says, "my hand is yours." In return, Alina gifts Inej one of her daggers, essentially turning Inej into her faithful blade. It's one of the most moving displays of affection in Shadow and Bone season 1 and highlights the personal beliefs that guide the women during an especially difficult time: faith and trust in the process, faith and trust in each other.