There's a few reasons.
1) I know I'm going to have to fight tooth and claw to get through transition. In this case, my choice is living a lie or becoming a fighter, and I think I'd rather die fighting than as a coward.
2) In RPGs I usually play as the melee character (close-ranged fighter), and if there are classes they are often called Fighters or Warriors. I prefer Fighter personally, as it's reminiscent of Final Fantasy and Dungeons and Dragons, not to mention it's generally more neutral than a Knight or a Paladin.
Also I didn't add "Sadie" to the end until recently. I prefer when people call me Sadie than Fighter, since it kind of sounds weird to actually be CALLED that. It fits better as a type of title I think

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