Boggs is one of the few people Katniss comes to trust in District 13. He seems to be on her side more often than not. When she, Gale, and Beetee, among others, are debating inhumane and humane war strategies, for example, Boggs reminds them of the importance of compassion and basic survival: "There's always the larger question: If we engage in that type of war with the Capitol, would there be any human life left?" (6.63). In other words, he's not as trigger-happy as some of the other rebels.
Boggs takes care of Katniss in dangerous situations and she can count on him to tell her the truth. For example, Katniss turns to Boggs when she's trying to figure out if Coin has sent Peeta to kill her. He tells her that Coin is afraid of her:
"You're a threat. You're the face of the rebellion. You may have more influence than any other single person. […] Outwardly, the most you've ever done is tolerated her." (19.13)
Boggs encourages Katniss to watch her back and take care of herself when no one else can. After this conversation, Katniss feels closer to him than ever.
Soon after this conversation, Boggs is mortally wounded in combat. As he's dying, he keeps his head and focuses on his primary goal, which is leaving Katniss in charge of their Squad. Boggs is about the bravest person you might ever meet. Who else would have that much presence of mind after just losing both legs to a bomb blast and starting to bleed out?
Even after Boggs completes this complicated "transfer of prime security clearance" (20.6), he's still worried about Katniss. His last words are for Katniss: "Don't trust them. Don't go back. Kill Peeta. Do what you came to do" (20.18). His dying thoughts are for Katniss and the cause.