Characters
Half-Life has several different characters, both good and bad. Many of the characters listed are helpful and are your fellow co-workers. Whether or not your help them is up to you, and whether they live or die is also up to you.
Scientists
These guys are your fellow co-workers and friends who are stuck in the middle of the Black Mesa Incident. Scientists may be frightened and cowardly, but your survival depends on them.
Scientists can open locked doors, follow you around, provide you with information, and even heal you. They should not be harmed and treated with extreme care.
There are different types of scientists, but you can notice them easily with their white coat, white pants, blue shirt, and red tie. They cannot use weapons and do not drop any items when killed, so keep that in mind.
In the PS2 version of Half-Life and in the High Definition Pack (explained here), scientists are much more detailed and have brown pants instead of white. They also have different death animations and positions.
In some cases, you cannot finish a stage without a scientist doing something, such as unlocking a door, so make sure you always keep your eye on them.
Security Guards
Security guards will be one of your best freinds during the game. They are highly trained guards that work for the Black Mesa Facility. Your survival also depends on these guys.
Guards can follow you around, give you covering fire, provide you with information, and open locked doors. Like scientists, they should be treated with extreme care. Note, however, that the guards will attack you if you attack them or a scientist.
All of the security guards look the same, and you can notice them with their helmets, body armor, and blue clothes. Security guards only carry Glock pistols around, and will drop them once killed.
In the PS2 version of Half-Life and in the High Definition Pack (explained more here), security guards are twice as detailed but still wear the same kind of clothes. They also have different death animations and positions.
Like scientists, security guards may be critical for some missions like unlocking a door, so keep your eye on them. They can defend themselves pretty well against smaller enemies, but are defenseless against soldiers and assassins.