Title pretty much gives it away:

Katana of Fire:

"Attached character gets +1 for each Fire card you control"

Rules Reference on Card Controll (P.5):

"A player controls the cards in his or her out-of-play game areas (such as the hand, the dynasty and conflict decks, and the dynasty and conflict discard piles)."

Rules Reference on Roles (P.15):

"Role cards are not considered in play. Their text affects the game state from the out-of-play area while they are active beside a player’s stronghold."

Some clarification on this? Or is there a rule that says, when cards reference other cards you control, they have to be in play`? Otherwise Katana of Fire would get +1 for every Fire Card in your Deck and your role. I'm sure this is not intended.

Litteraly on the first page of rules, under Ability:

"Card abilities only interact with, and can only target, cards that are in play, unless the ability specifically refers to an out-of-play area or element. Card abilities on characters, attachments, holdings, strongholds, and provinces can only be initiated or affect the game while they are in play unless the ability specifically refers to being used from an out-of-play area, or require that the card be out of play for the ability to resolve. Event cards and role cards implicitly interact with the game from an out-of-play area, as established by the rules of their cardtypes"

Thanks for the clarification.