The Niki/Jessica Sanders Character Arc
There is this confusion concerning the Niki and Jessica storyline is deciphering between the two and their abilities. Are Niki and Jessica two completely separate people, or is Niki channeling the Jessica personality from her traumatized past? If it is the latter, what causes the initial mirror-reflections to communicate? And how could the Jessica personality be sporting the helix-tattoo while Niki doesn't? These are questions that fandom has asked, even argued, though there is still no canonical evidence explaining the mysterious alter egos.
One of the popular theories is that Niki is suffering from Dissociative Identity Disorder, where the symptoms do match up and offers a logical explanation for what her character experiences in the series. The alter ego of Jessica could also be in connection to her traumatized past, since Jessica died at the hands of their abusive father, and it's been confirmed that Niki hadn't remembered anything from that period of time where her father had been abusive. Perhaps the death of her sister allowed her to take on that different personality as a shield from herself, possibly without even knowing it. It's not uncommon for something like that to happen, as I recall in an episode of Criminal Minds where a serial killer had multiple personalities, one of them his own father due to the guilt over killing him. If this logical explanation serves underneath that, Niki feels guilt over Jessica's demise that she acquired that personality to deal with certain situations because Jessica had been the one taking the fall in order to protect her sister. All these are possibilities for explaining the alter egos, her blackouts when Jessica took control, and so forth.
However, since this is Heroes we're talking about I won't settle for simple explanations just yet regarding this particular character arc. In this universe, not everything is what it appears to be.
My first theory is that Jessica is, in fact, possessing Niki's body from beyond the grave. It's been made known that relatives/siblings of those with special abilities may have a power as well, and if Jessica and Niki were twins, as some speculate them to be, then there is a chance that Jessica did indeed have an ability. Whatever that ability may be I don't know, but the idea is certainly plausible. Going along with that, if Niki and Jessica were twins there's the twin-connection theory. The episode "Six Months Ago" was supposedly telling the beginnings where some of the characters began to notice their abilities manifesting, and once Hal mentioned about Niki not remembering what happened (i.e. the childhood abuse), and immediately after saying this Niki looked into the mirror and that's where Jessica had appeared. Initially, those who agree with the DID theory would say that Niki's father appearing after all that time could've triggered the Jessica personality, however I think that, along with the rest of our Heroes, this was Jessica's first appearance in terms of her manifesting via whatever ability to possess her sister's body.
I also think that Jessica still wants to protect her sister. Everything we've seen so far was Jessica's attempt to protect Niki and her family, specifically Micah, even wanting to teach Niki the meaning of being strong and confident by using the superhuman strength, which Niki was hardly using. Her wanting to frame DL was possibly her resentment and contempt for men in general. She probably thought she was doing Niki a favor in that sense.
Another theory, which someone had pointed out in a theories discussion thread I read, is the connection with the mirror reflections. There are speculations that when one personality has been repressed, they actually go into a different reality while the other is dominating the body for that time, which is what the mirror represents. Almost like an Alice Through The Looking Glass perspective. Perhaps Jessica had "died" in the real world, but her ability allows her to exist in a different reality and once she figured out the extent of that ability six months prior she was then able to co-exist with her sister by sharing the same body. But because they would have to communicate and do things that would affect the outcome in that other reality, that idea seems a little farfetched. So being trapped inside the mirror is more symbolic of who is dominating the body as the other helplessly looks from the other side.
There are many possibilities with explaining the two different personalities. I stand firm in believing it's actually Jessica using her own merging ability and share Niki's body, because it makes things more interesting than just a simple identity disorder.
The Sanders Family Storyline
I know in fandom people weren't too keen where the Sanders storyline was heading, and I unfortunately have to agree. It started off with strong potential and could've been interesting, however it began to slack off in the middle with little development until the last two episodes of the season. The show focused more on the primary main characters and everyone else became secondary. I felt that Niki's storyline should've expanded further to connect with the main plotline of the season, considering she had connections with Linderman. She met with Nathan Petrelli multiple times, Matt twice as Jessica, even spoke with Ando in the beginning of the season. They could've done some interesting things with her going outside of her family and working with other characters than limiting herself to those, which would've given her more reason to be involved in the finale fight.
However this show, particularly the first season, was all about family and self-discovery. Every character struggled with personal relations and as the season progressed so did those relationships, either growing apart or closer together because of them discovering certain things about themselves.
Niki was struggling as a single mother, but learned that she could be more than just that and with Jessica's help, she was able to find herself and her true potential. She'd repressed her ability because she had no confidence in herself, as I believe she's had that drilled into her since childhood that she wasn't "strong enough" that she began to believe it was true. But, with Jessica's support and, sometimes even taunting manner, she was able to bring out what Niki had all along. It's all with the power of the mind. Once Niki's confidence began to surface so did her ability, and in the finale when Niki was shocked to find that she finally had the strength she came to the actualization of who she is as opposed to what she used to be. Hence why when she looked into the mirror she only saw herself, not Jessica.
The reason why I like Niki is not only because of Ali Larter's ability to create these two totally different people
Then there's DL and Micah and where they fit in. Obviously, Micah was part of the plan of getting Nathan elected congressman, but was that it? Is that all Linderman wanted from Micah, because of this "bigger picture" the adult generation had foreseen? I have a feelings there's more to it than that, along with DL. Linderman is the main Puppet Master and had setup Niki and DL deliberately, so there's that.
Hopefully we'll get more answers in the second season concerning them, and more development with the Sanders family as a whole because so much could've been explored with them but it wasn't in the first season, which disappointed me.