Wondering if tag style (for tags with double-pair brackets) should be
*(OWNER)_(NOUN)
or
*(NOUN)_(OWNER) ?

existing *(OWNER)_(NOUN) tags:
1) 1 . . . adonis_(disgaea)_(character) . . . character.
2) 1 . . . bb_(fate)_(all) . . . character.
3) 1 . . . beth_(ikugo)_(character) . . . character.
4) 2 . . . blue_(capri-sans)_(character) . . . character.
5) 2 . . . buck_(disney)_(disambiguation) . . . character. wiki: buck_(disney)_(disambiguation)
6) 1 . . . byleth_(fire_emblem)_(male) . . . character.
7) 2 . . . carmen_(animal_crossing)_(mouse) . . . character. wiki: carmen_(animal_crossing)_(mouse)
8) 11 . . . corin_(luvbites)_(character) . . . character.
9) 1 . . . corrin_(fire_emblem)_(male) . . . character.
10) 2 . . . cu_chulainn_(fate)_(all) . . . character.
11) 2 . . . elizabeth_bathory_(fate)_(all) . . . character.
12) 6 . . . escort_(luvbites)_(character) . . . character.
13) 1 . . . faux_(uriko44)_(character) . . . character. (in this case, having _(character) at end may not be necessary)
14) 1 . . . harold_winston_(overwatch)_(character) . . . character.
15) 2 . . . jeanne_d'arc_(fate)_(all) . . . character.
16) 1 . . . marcus_(luvbites)_(character) . . . character.
17) 1 . . . mea_(ultrakirby123)_(character) . . . character.
18) 2 . . . medb_(fate)_(all) . . . character.
19) 1 . . . mordred_(fate)_(all) . . . character.
20) 1 . . . natalie_(housespets!)_(character) . . . character
21) 2 . . . nero_claudius_(fate)_(all) . . . character.
22) 2 . . . oda_nobunaga_(fate)_(all) . . . character.
23) 2 . . . okita_souji_(fate)_(all) . . . character.
24) 1 . . . robin_(fire_emblem)_(male) . . . character.
25) 1 . . . sans_(haventale)_(character) . . . character
26) 1 . . . sans_(reapertale)_(character) . . . character
27) 2 . . . scathach_(fate)_(all) . . . character.
28) 1 . . . sweetpea_(mickeymonster)_(fan_experiment) . . . character
29) 4 . . . tamamo_(fate)_(all) . . . character.
30) 1 . . . titan_(sonic_unleashed)_(character) . . . character
31) 1 . . . tony_(earthbound)_(character) . . . character

existing *(NOUN)_(OWNER) tags:
1) 1 . . . anne_bonny_(swimsuit_archer)_(fate) . . . character.
2) 2 . . . artoria_pendragon_(swimsuit_ruler)_(fate) . . . character.
3) 1 . . . atalanta_(alter)_(fate) . . . character.
4) 1 . . . gilgamesh_(caster)_(fate) . . . character.
5) 1 . . . justin_(mouse)(aaron) . . . character.
6) 5 . . . mark_(bear)_(aaron) . . . character.
7) 10 . . . mark_(horse)_(aaron) . . . character.
8) 1 . . . mary_read_(swimsuit_archer)_(fate) . . . character.
9) 2 . . . medusa_(lancer)_(fate) . . . character.
10) 1 . . . mojo_(lombax)_(mojoskunk) . . . character
11) 5 . . . mojo_(skunk)_(mojoskunk) . . . character
12) 13 . . . mojo_(skunkbax)_(mojoskunk) . . . character
13) 1 . . . morgan_(cat)_(shikakaka) . . . character
14) 2 . . . morgan_(kashiroo)_(shikakaka) . . . character
15) 1 . . . mox_(horse)_(the_mox) . . . character
16) 1 . . . nero_claudius_(bride)_(fate) . . . character.
17) 1 . . . sparkplug_(dog)_(transformers_animated) . . . character
18) 1 . . . tamamo_no_mae_(swimsuit_lancer)_(fate) . . . character.

existing (COPYRIGHT)_(CREATOR) tags (i think)
1) 1 . . . bowhead_whale_(kemono_friends)_(yoshida_hideyuki) . . . character
2) 1 . . . tree_sparrow_(kemono_friends)_(yoshida_hideyuki) . . . character

* * *

Below copied and adapted from searching for "*(*)*(*)*"
https://e621.net/tag?name=*%28*%29*%28*%29*&type=&order=name

all of these look cancerous, and like they shouldn't exist. It's completely over complicating things, or using brackets to descriptive tag things. Of course the duplex brackets seem to be klorpa for the most part, heeeeh. Just consider them tags that should be simplified as far as I'm concerned.

Overall I'd say double brackets should be avoided completely. I agree with Versperus that they should be considered tags that need simplifying.

Characters should be character_name_(character_owner) and artists should be just the artist tag, only using _(artist) if there's a non-artist tag that's the same.

As an example of simplifying some of them:

applejack_(mlp)_(g1)
applejack_(mlp)_(g2)
[..]

Should probably be applejack_(mlp) with alternate_color/alternate_form as additional tags, or at worst applejack_g1_(mlp)

should be carmen_(animal_crossing) with the alternate_species tag, presuming they're not normally a mouse.

and byleth_(fire_emblem)_(male) would just be byleth_(fire_emblem) and the male tag.

Quick edit:
Just thought I'd leave a link to the tags section, showing other alternate_* tags:
https://e621.net/tag?name=alternate_*

Yeah most of these are unneeded bloat in my opinion.

For character names the general rule of thumb for formatting is:

For non-OCs:

(character's full name)

If the character's name is the same as a/their series' name (like Donkey Kong):

(character's name)_(character)

If their full name isn't know or it isn't unique:

(character's name)_(series)

This that still isn't unique:

(character's name)_(series)_(description)

Looking at your list, this one would be used for the Carmens from Animal Crossing since there is a rabbit Carmen and a mouse Carmen.

For OCs:

If their full name is unique:

(character's name)

If their full name isn't unique (This one should be used the vast majority of the time in my opinion):

(character's name)_(creator's/owner's name)

And if they have the same name as the owner/creator:

(character's name)_(character)

Basically,

*_(character)

formatting should only be used to disambiguate

crusty_fire said:
And if they have the same name as the owner/creator:

(character's name)_(character)

If an artist and character have the same name, the character would still be character_name_(character_owner), with the artist just being the artist tag.

Pupslut said:
carmen_(animal_crossing)_(mouse) should be carmen_(animal_crossing) with the alternate_species tag, presuming they're not normally a mouse.

there's actually two completely different characters who use the name "Carmen" in the english translations,
one who was in the original Animal Forest for the n64 and then didn't make any appearances Wild World onward)
and another character introduced in City Folk and has made an appearance in every game since then.

The character-name_(species)_(owner) tags are for when an owner has 2(or more) characters with same name, but different species.

forum #265849 : thread "What format for character tags when same owner has 2 (or more) characters with same name?"

darryus said:
there's actually two completely different characters who use the name "Carmen" in the english translations,
one who was in the original Animal Forest for the n64 and then didn't make any appearances Wild World onward)
and another character introduced in City Folk and has made an appearance in every game since then.

How does this game universe work, if people were to discuss on both Carmens in the same dialogue? Like the Smurfs universe that no-one is even trying to distinguish who is who? (The smurf smurffed the smurf, causing the smurf to smurf the whole smurf!) Or do they specify like: "Carmen The Mouse visited Carmen The Rabbit yesterday."

In the latter case, I'd be voting that there'd be tags Carmen_The_Rabbit_(Animal_Crossing) and Carmen_The_Mouse_(Animal_Crossing)

I think these bracket extensions are there only to disambiguate things with equal names. That is to say, no-one should search for Animal Crossing characters with the thought of assuming all characters have "_(Animal_Crossing)" in their character tag. Rather the search should be done with the copyright tag animal_crossing.

Thereby, there shouldn't be need for particular systematics (except for the sake of aesthetics), and it would be a standard that there is only one bracket specification at the end. In case of the AC Carmens, these could be Carmen_(Animal_Crossing_Mouse) and Carmen_(Animal_Crossing_Rabbit), or if the mouse is more significant character in the game, it could be just Carmen_(Animal_Crossing), as it is the rabbit that needs more often to be marked, when being discussed.

"Bob was doing this, and that. Then, as Bob was continuing doing this, in particular, all of a sudden, Bob From Next Door burst in. Bob was a bit puzzled of this, but then Bob From Next Door said he had taken the wrong door and left, and Bob was able to continue doing this and that." – This is more efficient than always talking about Bob From Our Office as well.