With an ensemble cast, many characters get lost in the story lines.
These characters get under-used and under-rated.
Perhaps the most under-rated character in the series was Rose.
She was one of the older castways. She was married to dentist, Bernard, who found happiness at a very late age in life. Bernard adored his wife, and would do anything to make her happy and safe.
But when Rose came down with terminal cancer, Bernard could not let go. Rose, through her own intelligence and faith, came to terms with her condition. Bernard could not; so he kept going to out to find miracle cures.
Since Rose had both the intelligence and faith to deal with life and death, she was the real crossroads between those themes in the series. Science vs. Faith. Life vs. Death. How do people cope with terrible news? Rose could have been the perfect sounding board for the other characters to gather keen advice.
Besides the one beach scene with Jack, where Rose was alone but content that she would find her husband again (to Jack's dismay), Rose was not used as a strong character. She was a worker bee at the camp site, taking on a distant motherly presence but without deep interaction with the other female characters.
In early theories, the Jack and Rose discussion where Rose was at peace with herself was a telling, important clue. In the purgatory theories, this was the moment where the viewers saw Rose being cancer free, no longer in pain (which is impossible). They believed that Rose was at peace because she knew that she had died - - - death frees the human body from its mortal pain - - - so she knew in her faith that she knew Bernard would be okay (even if he died, too). Knowledge was power on the island, and Rose's awareness of the situation of the other passengers souls could have given the writers the opportunity to create a duality. Rose never worried or cared about what the others were doing because she knew it did not matter - - - they were all dead. The other characters were unaware or refused to accept their demise. (Which is confirmed in the last season with the sideways universe arc).
Rose could have given sage advice throughout the series, but was placed in a secondary, background role. She did not get involved in missions because that would be out of character. She did not push herself into leadership roles because she knew that each person had to find out for themselves what has happened to them.
Rose was a strong enough character to carry a secondary story line (about lost souls in purgatory) against the background of personality clashes between Jack, Locke and the Others. She could have been the beach camp's Alpert.