A character is any sentient being in the RuneScape world. Characters fall into two categories: player characters (commonly referred to as "players"), and non-player characters (commonly referred to as a NPC, marked as a yellow dot on the Minimap).
Players are shown in different colours on the mini-map based on what they are in relation to the player. Players in white have no affiliation to the player, players in blue are either on the same team in a minigame activity or wear the same Team cape as the player, players in green are on the player's Friends List, and players in purple are in the player's same Clan Chat.
NPCs can be divided in attackable and non-attackable NPCs. Attackables are NPCs like monsters and some people like men, women and guards. Some NPCs can attack players if the player's Combat level is twice that of the monster or less. For example, if a player approaches a level 28 hobgoblin, said player will be attacked unless their is 57 Combat or higher. These monsters are said to be aggressive, and almost every boss and all monsters in the Wilderness are aggressive. Most monsters have combat levels just like players (exceptions are the Strange plant and the Security Guard), but many NPCs do not.
On the other hand, non-attackables are like, but are not limited to Shop assistants and guides. The Lumbridge Guide explains the difference also. These NPCs can usually converse with the player or carry out a certain action, such as bankers, who allow players to access their bank accounts. Most NPCs have a certain zone through which they can move freely (exceptions are Random event NPCs and certain quest-related NPCs, such as the King's messenger, who can appear anywhere).
There are many different possible 'looks' for your character. Many players do not care about their look as their characters are almost always seen wearing armour and especially bots who just use the default character setting consisting of a light skin tone, bald, and bearded.
In-game players are often named by their username by other players and by NPCs. NPCs sometimes give nicknames to players depending on their relations to them or by their descriptions or by their traditions or by any other methods not mentioned here. For example, upon completion of The Fremennik Trials quest, players are called by their Fremennik name. Another example is how the troll race give names depending on what they first ate as a child. Different races call players different names as well such as 'Mortal' and 'Human'. In most quests involved with the mainstream RuneScape storyline, players are frequently referred to as, the Adventurer, meaning that the player is in fact an adventurer or a chosen hero.