....She's A tier?! I knew it! She does have potential after all. :-D I say this because a lot of players who used her say she's not that great because of linearity, but you're not exactly supposed to just rushdown with her or just mindlessly use the K and 6K kick strings.
She never was a bad character to begin with. Always have been like a Top 15 or right outside of it. She has everything that she needs in the game. Just that she's also the most studied character in the game because she's one of the most popular. She gets predictable when you're not fully utilizing her strings & pressure options. One thing you have to understand with her no matter playing offensively or defensively is you can't get caught standing still with her. And you have to be willing to take some risks. There is a such thing as taking a good risk: Like 66K-K but getting held for being safe at -3. The hold doesn't do a lot of damage for the defender so that's what she wants if they react to it rather than block for safety.
She's a character that puts more emphasis on offensive matters, but also has good defensive options being able to counter strike, poke, with parries & an expert holds. One of the best wall games in the game, one of the best ceiling games in the game; people knock her & others out who can utilize the ceiling because they all do a shit ton of damage.
Her being unsafe stops her from going any higher & others have better range options (though she has her mobility to move around).
EDIT: Yeah mindlessly mashing strings is one way to die. In order to play Kasumi, you need to be good at string delaying & free canceling while understanding her longest reaching tool (66K).