the Henry/Charles dynamic is a fascinating case study for loyalty, but it's way more complicated than 'they're loyal to each other.' I mean, in the games themselves, Charles's loyalty is primarily to the government. He'll help Henry, sure, but his ultimate mission is capturing the Toppats. Henry's 'loyalty' is mostly transactional—he needs help escaping, and Charles is a useful asset. The 'Distinguished Consequentialist' ending where Henry betrays him kinda proves that baseline.
Where the fanfiction really runs with it is by flipping that script. Writers explore a version where Charles chooses Henry over his duty, or where Henry, after all his betrayals and chaotic neutral choices, learns what it means to actually be loyal to one person. It's about earning that trust, which isn't really present in the source material. A good slow-burn will have Charles wrestling with his oath versus his feelings, and Henry learning to value someone enough not to ghost them after the heist.
A lot of fics dig into the aftermath of 'Valiant Hero,' where Charles presumably died. That's the ultimate loyalty test, right? Grief, survivor's guilt, Henry maybe realizing too late what that friendship meant. It creates this angsty foundation where loyalty becomes about honoring a memory or making amends to a ghost, which is a powerful, if painful, theme to explore. You see a lot of fix-its born from that, where the driving force is a desperate, loyal attempt to rewrite the ending.