In comics, childhood friends are quite common. These are the types of friendships the started off when they were both kids and stayed strong even at adulthood. Sometimes these friends have always been together while other times they may not see each other as often as they use to (one moved away, their lives had two different focuses, or they just slowly drifted away). There are three major outcomes of a childhood friends; Romantic Interest, Personal Enemy, or Important Ally.
This is an evolution of childhood friendship between a boy and girl. Usually they were really good friends as kids and one promised to marry the other and they both grew up harboring feeling for the other (it may be one-sided at first). This type may also be found in the next two, more often in Important Ally but it may also be in Personal Enemy.
They were good friends as kids but as they grew up something made them have a disagreement about a moral subject. This is what happens when two childhood friends become enemies. In the superhero community they can either know the other's identity or have no idea. If they know the identity of the other it will come up in talks between the two of them in-costume. But if they don't know that a childhood friend is their enemy in disguise then they may still actually be very good friends when out of costume. This idea first sparked in an expanded origin of Superman in "Superboy" Comics, where Superboy and Lex were the best of friends, until one faithful day where Lex tried to find a cure for Kryptonite poisoning for his pal, resulting in a fire / explosion that he blamed entirely on Superboy, which caused him to become permanently bald and possibly sociopathic.
However, this type has several variations. Sometimes only one knows of the other's identity. Maybe they are enemies out-of-costume too. There may also be a flip-flop where they are good friends in-costume but enemies out of costume.
*see Hush & Batman.
If the childhood friend ever finds out the hero's secret identity they usually become a huge support in their heroic career (sometimes even villainous). They tend to be on the sidelines offering information, equipment, advice, and sometimes investigating somewhere they can't be. But if the childhood friend also has superpowers they tend to fight along side the hero on several occasions.
*see Gear & Static.