Kaname Chidori originally lived in the United States with her whole family as a kid. Unfortunately, tensions grew in her family, and Chidori and her mother moved back to Japan to live on their own. However, Kaname lives alone now, as her mother died before the beginning of Full Metal Panic. Kaname apparently is still in contact with her sister Ayame who still lives with their father Shunya but isn’t on speaking terms with her dad. Unbeknownst to Kaname before the beginning of Full Metal Panic, she holds incredible scientific knowledge in her mind. Kaname can’t access it though, but it is possible for someone with the proper knowledge to bring it out of her. That’s why she needs protection, even if she doesn’t know she needs it. The secret mercenary organization Mythril sends Sousuke Sagara, a teenage soldier, to guard Kaname in secret.
Kaname isn’t the most popular girl around. Despite being a very kind, intelligent, and beautiful girl, she’s also fairly outspoken and isn’t afraid to speak her mind. Combined with her past history of living in the US, this makes her very intimidating to your average Japanese student. This has isolated Kaname from her peers, but she’s not too upset about that. She gets along fine with the few close friends she does have and will do anything for them.
That being said, it’s not as if Kaname exactly enjoys the isolation either. Maybe that’s why she’s willing to put up with Sousuke’s antics around Jindai High School as much as she does. At the very least, Sousuke’s hovering gives Kaname some company. Thanks to Sousuke, Kaname has fully revealed herself to the world as a bit of a firebrand. In Kaname’s defense though, it’s not like she can help it when the guy who has latched himself onto her is pulling handguns on people just for approaching while holding a pair of scissors.
Sousuke Sagara has become an incredibly important person to Kaname by sheer virtue of proximity. Even if his PTSD stresses her out to no end, in a weird way Kaname feels safest when she’s around Sousuke. Perhaps it’s because when all is said and done, Kaname knows that she is the one person who Sousuke trusts unquestionably. Kaname acts as something of a teacher for Sousuke by showing him that civilian life has its benefits over living in a state of constant fear and aggression. She only gets angry because she loves Sousuke and worries constantly about his well-being.
Beyond that, Kaname only has one really close friend in Kyoko Tokiwa. Kyoko understands Kaname’s feelings for Sousuke better than Kaname even does, and attempts to play matchmaker for them throughout the course of the series. When Kaname is reunited with her junior high crush in Full Metal Panic? Fumoffu, Kyoko even informs Sousuke about their meeting and encourages him to make a move for Kaname. Likewise, Kyoko is something of a weak point for Kaname. In Invisible War, Amalgam agents take Kyoko hostage to manipulate Kaname into coming with them. The two are besties for life, and nothing will ever change that.
Kaname is our human element to ease us into the story of Fully Metal. Sousuke may have more of a character, but his history makes him so distant from anyone your average anime watcher would ever know that he’s too distant for us to really relate to him. Kaname grounds us in reality. She’s the one who’s speaking what we’re all thinking when Sousuke blows up the school for the 5th time. Without Kaname, Full Metal Panic would just be a dry military anime discussing international politics that go way beyond our heads. We need Kaname to give us someone to care for, which in turn gets us closer to Sousuke’s struggles.