Between flashbacks, flash forwards, and people on the island, there are a lot of minor Lost characters to keep track of.
The people in this group survived the crash of Oceanic Flight 815 along with our Losties on September 22, 2004.
These are the people who were on Oceanic Flight 815, but either didn't survive the crash, or died shortly after. Included in this group is Gary Troup, the fictional author of Bad Twin.
We've met a lot of minor Lost characters through our main characters' flashbacks. This is how we got to know the Losties better and we also realized that their lives were intertwined before coming to the island.
These characters were seen after the Oceanic 6 were off the island. Again, the lives of our characters are very intertwined, even though they don't mean for them to be.
These characters are seen off the island in the flash-sideways. Many of these characters we've seen before in flashbacks or flash forwards.
The Others are probably the scariest group as a whole. They've terrorized our Losties from the beginning, with Ethan and Goodwin living among the survivors, not to mention the time Mr. Friendly stole Walt off the raft (Ep: 1x24, Exodus Part 2).
We really got to know a lot about Dharma and the people that lived there during season 5 when some of our Losties lived among them.
I can't promise that an Other or two didn't sneak onto this list. You never know about those sneaky Others. These are the people that as-far-as-we-know aren't Others, but were on the island at some point.
There was a group of people that came close to the island on the freighter. Some never left the freighter, and some came to the island. Some were nice, and some were evil.
In 2007, Ajira Flight 316 crashed on the island. Most of the Oceanic 6 were on board, as well as other people.