Spoilers for Fuuko's arc of the Clannad anime will follow.
Fuuko is actually a comatose young girl who was hit by a car on the way home from her high school's welcome ceremony. As such she exists as two different 'forms'. Her real body is of course in a coma and remains in hospital. The other Fuuko most people will interact with however is the spirit (ikiryo) incarnation of her. Despite being a spirit, she acts like any other high school girl. She is capable of wearing different clothing, taking baths, her reflection probably appears in mirrors too! She can't walk through walls or any other sort of 'spirit' stereotypes, or perhaps that's more in line with ghosts... Anyway.
Fuuko is fully aware of her condition, though if asked about it she will not say much. There might be a few vague hints as to what she is doing and her current condition, but otherwise she will not outright say 'I'm actually in a coma'.
Fuuko's coma and the effects it has on her.
When Fuuko's condition is good and her real body is healthy, it reflects on her spirit self positively. She's free to wander around and be a normal girl. Should Fuuko ever wake up from her coma, then her spirit self will disappear and her real body will of course act in her stead. As she is a spirit, Fuuko will be perpetually 15 years old and cannot age. However, her real body will still age by normal standards, and is currently 17 years old.
Should Fuuko's condition take a turn for a worse then it will negatively impact on her spirit self. Whilst she won't disappear, people will start to forget about her. Depending on the severity of her condition, people might just forget bits and pieces of conversation with her, to not recognising Fuuko the next time they see her, or possibly being unable to see Fuuko at all. Even if you cannot see her, Fuuko will always be around in her spirit form as long as her real body remains under a coma. Should Fuuko's real body die, then her spirit self would also die.
Furthermore, should anyone see Fuuko's real body, they will instantly forget everything they ever knew about Fuuko's spirit self. There might be a few lingering memories and a sense a familiarity when they see Fuuko's spirit self again, but otherwise a person completely forgets anything major relating to Fuuko's spirit. Should someone realise it's impossible for a girl who is comatose to be running around, acknowledging the fact Fuuko is in a coma after seeing her body, they will be unable to see Fuuko. At all.
How will this effect you?
My character has met Fuuko's spirit first, what do?: That's fine. There's nothing different to meeting a normal person in this case.
My character has met Fuuko's real body first, what do?: They will be unable to see Fuuko's spirit self sadly.
My character saw Fuuko's real body first, is there any way for them to interact?: Not directly. It's possible for someone who knows of Fuuko's spirit form to talk to someone aware of her comatose state however. (Canon example: Tomoya telling Kouko how happy Fuuko is for her to get married and reassuring her she should put her own happiness first, despite Kouko knowing her sister is still in hospital.)
My character has learnt about Fuuko's condition without actually seeing her real body, what do?: This is fine too. Good for them for finding out this way. There might be a time when her condition worsens and they forget about Fuuko that way, but otherwise it'll be the same as interacting with a normal person, only you know Fuuko's real identity.
What if Fuuko's spirit self and comatose self are in the same room?: You would be unable to see the spirit and only Fuuko's real body. This is unlikely to happen much however.
Fuuko's condition is currently poor, what will happen if I meet her now?: You might not be even able to see her until her condition improves. Those who already know Fuuko's spirit might forget details of past conversations or time spent with her. If her condition becomes critical, they might not be able to see Fuuko and furthermore won't recall anything about her until her condition improves.
My character has forgotten Fuuko, can they remember?: Yes and no. If they have forgotten a few details, they might remember again later after spending time with Fuuko can her condition improves. If they have forgotten her completely however, such as from her condition worsening, and are unable to see Fuuko, it is possible for them to see her again after they are reminded of her existence. This could happen by recognising something which belongs to Fuuko, or recalling her name from some other prompt. (Canon example: Nagisa and Tomoya remembering Fuuko after being reminded of Kouko's wedding.)
I'll probably add more questions, details and such as time passes, but I wanted some basic information down about Fuuko's condition. If you think I have missed or skipped over something regarding Fuuko's condition and how it impacts her spirit self, or if you have a question I have not addressed, please leave a comment below and I will get back to you.