(It seems like this is the last remotely think-y post that can be teased out of me in a while. Oh well. I can't decide if this is a good or bad thing.)
I look at stuff I enjoy reading and/or watching, and find majority of them dealing with secret identity/other identity, whether by real physical transformation or just plain old disguise or simply a case of mistaken identity--where character X is thought to be someone else, whether by deliberate deception (not so fond of this type for mistaken identity), or something that simply happens because the events sort of lead to it.
Anyway, got inspired by some comments in a comm; at least a few things I like about secret identity stories/series. This post is not series-specific, hopefully, though, to be fair, I would have to note that the comm I mention is fandom-specific.
1.) Character X's normal identity and their disguised identity seem to scream "different person", yet they are actually the same. Same yet different at the same time, I find that dual nature fascinating, especially when I (the audience) know who they really are, and the other characters don't realize that fact and treat the two identities differently.
2.) Perhaps following from 1), character X is usually braver in their disguised identity (and sometimes more reckless, which might not be so good a thing, depending on how one looks at it), because he/she is wearing a mask of sort, so doesn't feel so naked to the world, I suppose. This results to him/her usually having to confess to the object of his/her affection the feelings of his/her real identity. Then, of course, the object of his/her confession usually gets flustered (and sometimes upset) as to why she/he can't hear this (the presumably intimated feeling) from character X him/herself. Since I (the audience) know it's the same person, I just feel tickled, and somehow hope that the object of affection is smart enough to realize the truth. (As illogical as it is (since it seems so obvious to me), the object of affection is usually kind of slow about catching on. I'm sad sometimes because these people seem to be rather intelligent otherwise.)
3.) People keep remarking how they have never seen character X and their alter-ego around together at the same time. I usually either try not to laugh at the excuse(s) character X comes up with (with apologies to character X), or am blown away by his/her resourcefulness in dodging the issue (mostly it's the former case, though, more is the pity).
4.) By the same token as 2), character X gets to hear secret confession(s) from the object of his/her affection. (And can plan their strategies accordingly. ;P) This time, character X becomes flustered and almost always does things to at least compromise their secret identity (inadvertently replying as his/her other self, for instance*) and the object of affection, as always, still has no clue of the real identity of the person she/he is confessing to.
*It is especially amusing when character X uses different kinds of speech in the two identities. I'm thinking of Japanese fandom mostly, when males and females (in informal settings) usually speak in different speech patterns. (I suppose kids and adults do speak differently, too, but I can think of only one fandom where the dual identity thing is applicable.) I mean, I always very much like seeing a male character saying 'ore' (and 'ze' 'zo' or other typical rough male speech markers) while he's a girl, either physically via transformation, or pretending to be one. By the same token, it's fun to see a guy (whose real identity is a girl) or a girl who disguises herself as a guy forgets herself and uses 'atashi' and other typical soft feminine speech markers. It is a little difficult to explain, but, put simply, I guess I expect a feminine-looking girl to speak softly and a masculine-looking guy to speak, well, roughly. (This is shallow, I know, and there is an exception to every rule, but to keep things non-complicated, I would rather just focus on the absolutes. They should provide reasonably accurate guidelines, at least.) When the role is reversed, it becomes jarring, because it's defying my expectation--fortunately, however, in the way that usually makes me sit up and take notice, not in the way that takes me out of the story.
Conclusion to all this? I have too much fun seeing character X trying to keep his/her two identities straight, be it by the language or manners or anything else. (My favorite character has two cell phones--for each of his identities. :)
Now, not that it's related to this post or anything, I kind of lose track of what I want to say in a 'characters I dislike' mini essay (if it can even be remotely called that). It doesn't help that it's currently on paper and I can't exactly follow my own handwriting from a few months before. (I can read it; just not sure which direction(s) to go next since there must be some omitted arrows in there somewhere.)