Luke and Leia appear in multiple flashbacks, including at least one scene where Leia is being trained or is discussing Jedi training with Luke. The outcome of the training is uncertain. It is also unclear if Carrie Fisher and Mark Hamill filmed these scenes and will be de-aged, or if they are portrayed by younger actors and CG'd like Leia and Tarkin in Rogue One. My source has heard nothing about about Han appearing in any flashbacks.
 
Leia's role as Resistance leader is being said to be of a similar in size to her TFA role, perhaps even smaller, though her flashback scenes make her seem more present in the film. No confirmation on whether Leia lives or dies in the film.
 
There are solid red First Order Troopers, yet their role is unclear at this point in time.
 
Nien Nunb appears in an identical costume from the previous films and has a larger role than either TFA or TLJ.
 
The Knights of Ren do appear and each have a distinct weapon of choice as has been previously seen in TFA and the concept art for that film.
 
We don't have any confirmation that the any of the Knights previously seen in TFA will or won't be appearing in IX.
 
Keri Russell's character is a bounty hunter with an allegiance that is unclear to my source. Keri's character wears a purple (nearly magenta) jumpsuit with bronze/gold accessories, including an elongated helmet.
 
Think about a fancier version of Zam Wesell and my source says that will get you pretty close.
 
Naomi Ackie's character is supposedly Finn's sister.
 
Moderator's note: This point was presented to my source as factual like all the other parts of this post, but I believe it might be wise to take this one with a grain of salt until other sources can corroborate this particular bit of information.
 
C-3PO has a scene where he holds military gear. It's unclear whether he actually fires any weapons though.
 
My source doesn't recall exact details but 3PO appeared to have a bag or net (not unlike the Ewok trap net) and what he/she believes to be Chewie's bowcaster.

Luke and Leia appear in multiple flashbacks, including at least one scene where Leia is being trained or is discussing Jedi training with Luke. The outcome of the training is uncertain. It is also unclear if Carrie Fisher and Mark Hamill filmed these scenes and will be de-aged, or if they are portrayed by younger actors and CG'd like Leia and Tarkin in Rogue One. My source has heard nothing about about Han appearing in any flashbacks.
 
Leia's role as Resistance leader is being said to be of a similar in size to her TFA role, perhaps even smaller, though her flashback scenes make her seem more present in the film. No confirmation on whether Leia lives or dies in the film.
 
There are solid red First Order Troopers, yet their role is unclear at this point in time.
 
Nien Nunb appears in an identical costume from the previous films and has a larger role than either TFA or TLJ.
 
The Knights of Ren do appear and each have a distinct weapon of choice as has been previously seen in TFA and the concept art for that film.
 
We don't have any confirmation that the any of the Knights previously seen in TFA will or won't be appearing in IX.
 
Keri Russell's character is a bounty hunter with an allegiance that is unclear to my source. Keri's character wears a purple (nearly magenta) jumpsuit with bronze/gold accessories, including an elongated helmet.
 
Think about a fancier version of Zam Wesell and my source says that will get you pretty close.
 
Naomi Ackie's character is supposedly Finn's sister.
 
Moderator's note: This point was presented to my source as factual like all the other parts of this post, but I believe it might be wise to take this one with a grain of salt until other sources can corroborate this particular bit of information.
 
C-3PO has a scene where he holds military gear. It's unclear whether he actually fires any weapons though.
 
My source doesn't recall exact details but 3PO appeared to have a bag or net (not unlike the Ewok trap net) and what he/she believes to be Chewie's bowcaster.