Something that's really easy to miss when you don't read One Piece in Japanese is Oda's use of kanji in character names, specifically how rare it is.
The vast majority of actual given names in One Piece are written in katakana, like every Strawhat's given name: ルフィ (Luffy) ゾロ (Zoro) ナミ (Nami) ウソップ (Usopp) サンジ (Sanji) チョッパー (Chopper) ロビン (Robin) フランキー/フラム (Franky/Fram) ブルック (Brook) ジンベー (Jinbe) and frankly like 90% of all other names too, maybe even more
There are also a few that are written in hiragana like たしぎ (Tashigi) くま (Kuma) おつる (O-Tsuru) くれは (Kureha) しのぶ (Shinobu) おでん (Oden) ひぐらし (Higurashi) はっちゃん (Hatchan) しらほし (Shirahoshi) にんじん (Ninjin/Carrot from Usopp's village) たまねぎ (Tamanegi/Onion) (but not ピーマン (Pepper/Piiman) whose name is written in katakana, probably because that Japanese word is also written in katakana). And that's pretty much all the actually memorable characters whose names are written in hiragana, or the ones I can remember at least (sorry if I forgot someone important)
^ Female characters are definitely overrepresented in that hiragana category (considering how many more male characters there are in One PIece than female ones) which makes sense since hiragana is sort of associated with women (somewhat). A lot of these are also just Japanese words. Tashigi is a kind of bird, Kuma is a bear, Tsuru is a crane, Shinobu means "to hide", Higurashi is a cicada, Oden is a food, Hatchan is a cutesy variation on "hachi" which means "eight", Shirahoshi means "white star", Ninjin is carrot and Tamanegi is onion. Kureha's name is a bit ambiguous, it is apparently just a woman's name in Japanese, but it's also probably a reference to a Japanese pharmaceutical company, which itself gets its name from a village.
いっぽんマツ (Ipponmatsu) and うるティ (Ulti) are both a mix of hiragana and katakana. Ipponmatsu means "solitary maple tree" but Ulti is just a Hungarian card game and idk why it's partly in hiragana, unless it's just to show how wacky she is.
The big exception to the no kanji rule are the Wano characters, like 錦えもん (Kin'emon) モモの助 (Momonosuke) 雷ぞう (Raizo) 傳ジロー (Denjiro) 康イエ (Yasuie) カン十郎 (Kanjuro) 菊の丞 (Kikunojo) アシュラ童子 (Ashura Doji), although you might notice that most of these still use either hiragana or katakana in addition to the kanji, which is a bit unusual (except maybe for Kiku because the "no" being written in hiragana is pretty normal I think)
The only names I can think of that are written entirely in kanji are 日和 (Hiyori) and 玉 (Tama) and 鶴女 (Tsurujo). This also applies to Hiyori's Oiran name 小紫 (Komurasaki). There's also Denjiro's fake name 狂死郎 (Kyoshiro) and Ashura Doji's alias 酒天丸 (Shutenmaru). There are probably others too though, I can't be bothered to check all the minor characters in Wano.
There are still plenty of Wano characters whose names are just written in katakana, like トキ (Toki, although I guess she did grow up outside of Wano) トコ (Toko) スキヤキ (Sukiyaki) オロチ (Orochi) etc. I know I'm not listing enough of katakana names to fully demonstrate how common they are, but in my defence if I did start listing them it just would never end. Even in Wano katakana names are pretty common.
The surnames in Wano are all in kanji, though: 光月 (Kozuki) 霜月 (Shimotsuki) 黒炭 (Kurozumi) etc.
The interesting thing is that epithets like "Whitebeard" (白ひげ) and "Red-Haired" (赤髪) are much more likely to contain kanji. Even Luffy's 麦わら (Mugiwara) is partly in kanji. It makes sense because these epithets are supposed to be descriptive so of course they're just written as actual words. It's actually kind of a good trick to spot epithets and codenames as opposed to actual given names (aside from the aforementioned Wano characters). Like the admirals: 赤犬 (Akainu) versus サカズキ (Sakazuki), 青キジ (Aokiji) versus クザン (Kuzan), 黄猿 (Kizaru) versus ボルサリーノ (Borsalino)... etc. They all work the same way, including Fujitora and Ryokugyu and also all the "admiral candidates" with similar naming schemes.
(Arguably I think the more consistent thing would be to translate all the admiral codenames: Red Dog, Blue Pheasant, Yellow Monkey, etc. just like the pirate epithets are translated (like Whitebeard, Straw Hat, etc.))
There's also cases like タマゴ男爵 (Baron Tamago) where there's a title in the name and that title is written in kanji. This also applies to the "saint" in the names of Celestial Dragons, like チャルロス聖 (Saint Charlos) and シャルリア宮 (Saint(ess) Sharlia). And then there are characters who are only known by their title, like Woop Slap, whose name is never mentioned in the manga so he's always only referred to as 村長 (sonchō, village leader), same with the Shandrian chief who is only ever called "chieftain" (酋長 shūchō), etc.
Then we have バンダー・デッケン九世 (Vander Decken IX) リクドルド３世 (Riku Doldo III) and エリザベローⅡ世 (Elizabello II) where the generational number is written in kanji. Or western numeral and kanji.
And a rare example of a non-Wano character who has a kanji in their surname: ネギ熊まりあ (Negikuma Maria, AKA that bear-like onion lady from Drum Island, her surname literally means "onion bear") Her given name is also written in hiragana so you can add her to that list too, a rare exception where a non-Japanese name is written in hiragana (altho I guess Maria might be a popular name among Japanese Christians? Looking it up on Japanese Wikipedia, that hiragana spelling does seem to be somewhat common)
(The other Marias in One Piece are written in katakana: マリア・ナポレ (Maria Napole) and ブラックマリア (Black Maria))
As a sidenote, there are also some names which have Latin letters or Arabic numbers in them like Tボーン (T-Bone) ドクQ (Doc Q) ベビー５ (Baby Five), etc. Although unclear if they're actual given names or just nicknames/codenames/epithets. And then there are of course all the Baroque Works male agents, all of whose names are written in the same style: Mr.0, Mr.1, Mr.3, Mr.4, etc.... (Mr.2・ボン・クレー is a little bit of an exception lol; also their name is also written in kanji a couple of times: Mr.2・盆暮 (Bon Kure), but of course their legal name is written the usual way, in katakana: ベンサム (Bentham))
But are there any non-Wano characters (other than Negikuma Maria) whose real actual names, not nicknames/codenames/epithets, are written in kanji?
Well the only examples I can think of are 戦桃丸 (Sentomaru) and 軍子 (Gunko). And Gunko is highly debatable in terms of whether that's actually her "real name".
Sentomaru is clearly a reference to various Japanese folktales (especially Kintaro, but also Momotaro and probably others as well, and the name Sentomaru is a very folktale coded, I think) and Sentomaru himself kinda seems like a folktale character, being this super strong kid just randomly found in the forest.
Gunko literally just means like "army girl" or something. Technically 子 means "child" but more typically it's "girl", especially in names, especially when pronounced ko (compare アシュラ童子 (Ashura Doji) where it's pronounced ji instead)
The fact that there doesn't seem to be any good reason for Gunko's name to be in kanji makes it come off as extra weird and literal, at least to me. It really does feel like she was just lazily named for her purpose. "I want to make her into my child soldier so let's basically name her child soldier". (Obligatory note that this isn't the actual term for child soldier, that would be 子供兵 (kodomohei) or 少年兵 (shōnenhei) or in this case 少女兵 (shōjohei). Gunko is not a word that's used for that, as far as I know (I'm pretty sure tho), but it does kinda effectively convey the same idea. It also sounds kinda cutesy to me idk. Like literally I think "Army Girl" is a pretty good translation for the vibe? Although in English that doesn't sound like a given name. "Militarella"? "Waria"? "Battlette"? "Weaponie"? lmao)
Okay one more weird case: Zunesha. Zunesha's name is sometimes just written in katakana (ズニーシャ) but occasionally it's given the kanji 象主 with the pronunciation written in katakana as furigana. 象主 would be pronounced something like Zōnushi, which sounds a little bit like Zunesha but not exactly, and it means "elephant master" or "elephant guardian spirit" or something like that. I can't actually tell if it's even meant to be his name or if it's just a description of what he is.
Oda does sometimes use kanji with mismatched readings to convey extra meaning or nuance. See for example the One Piece itself, which is often written ひとつなぎの大秘宝 ("Great Hidden Treasure Connected As One" or something along those lines) with the furigana reading ワンピース (wanpīsu = One Piece). And the name "Zunesha" is likely based on Ganesha, or at least that seems likely.
Anyway, as for character names, I'm sure there are more interesting examples but that's kinda all I can think of and all the ones I can be bothered to check out. When it comes to the names of places or concepts in the One Piece world, that would take a whole other post to discuss.
I guess my conclusion is that I think there's something to these choices, at least in some cases. I feel like the choice to use so much katakana (and so many foreign names) is meant to give a sort of exotic vibe to the series, like it's clearly not set in Japan (despite some characters having Japanese names and despite there being a lot of Japanese cultural features all over the place), and then once they do go to Wano it can be a lot more obviously Japanese (but still with some weird elements). Writing Japanese names in katakana in particular might be a way to make them seem a bit more foreign while still being easy to remember and full of jokes and puns and themes.
The use of hiragana might be a bit vibes based and it seems to be mostly used for Japanese names and in particular for women and children and maybe also some childlike or cute or soft male characters?? idk (because hiragana is considered softer and cuter than katakana) Well, idk if Oden counts, but he is pretty childlike at least lol. Still, the hiragana names are pretty rare. I probably listed most of them in this post already.
This kinda gives the vibe that when kanji is used, it might be because the meaning is somehow significant (or the character is from Wano) or maybe Oda just wants to clarify the reference. With Sentomaru the meaning would not be obvious from the pronunciation so the kanji helps explain what the pieces are: 戦 = war, battle 桃 = peach 丸 = name suffix (in this case). The "peach" ties him to Momotaro, the Peach Boy. Otherwise you might interpret it as 戦闘丸 which would be kinda like "Battle Boy" since sentō also means "battle/fight/combat". And there are lots of other words that are read sentō as well; sen and tō are both very common kanji readings. I think Oda might have been wanting to make sure that people get his pun. (In addition to the folklore vibes maybe)
Gunko might also be somewhat confusing without the kanji. Not as confusing as Sentomaru, but still a bit ambiguous, since it's not really a real word or a real name. Although apparently gunko can also mean "war drum" (written 軍鼓). That's the only word I can find that's actually pronounced gunko (there are words that are pronounced gunkō with a long ō but not gunko with a short o.) But the use of kanji could also be a hint that Gunko isn't her real name.
And of course that's kinda the most interesting thing to me, the fact that kanji seems to often be a sign that the name is an epithet of some sort. When you see a name written in kanji, there's a very good chance that it's not the character's actual legal name, unless they're from Wano (but the Wano names are also generally very easy to recognise as old-fashioned Japanese names).
EDIT: added some colour-coding lol, hopefully it's not too obnoxious
