In short, Slashables, or manga with a relationship between two male leads that goes,
1 "I'll protect you with everything I am, and I'll never leave you behind."
2 But also won't take shit from each other and never hesitate to smack 'em over the head for being an idiot, every now and then.

I'm talking about along the lines of
Pandora Hearts fits both description.
Magi works too.

Kuroshitsuji, kinda fits the first.

I haven't even started reading these two but I sense there might be some of the element I'm looking for in
Hakkenden
Karneval

So, oh wise and magnificent fellow otakus, help me?

Yes, you will find something like that in both: Hakkenden and Karneval

Makai Ouji - Devils and Realist is a given.
Katekyo Hitman Reborn! is pretty much full of it.
Check out Tsubasa Reservoir Chronicle (Kurogane and Fai) if you have a lot of free time.
Also, I didn't read 07-Ghost and Nabari no Ou, but i think they may fit your request.
There are also sports series like Kuroko no Basket, Haikyuu!!, Prince of Tennis - they are always full of relationships between males.

Additionally browse through the category: Female Demographic with Male Lead

________________

Waaah, those are beautiful finds~
But I'm kinda feeling lazy wading into 475 results, so I'm just gonna wait for more recs, yes?
Anyway, Thank you so much!

*starts downloading*

I think you should check out this thread, since there are suggestions made there that'll fit your request very well. Maybe also the Pure subtext thread.

Get Backers is the epitome of this. They just don't make manga like this one anymore.
07-Ghost
Adekan
diece
Hunter x Hunter
Apocalypse no Toride
Junjou Karen na Oretachi da!
Haikyuu!!
Yorozuya Tokaidou Honpo
Ookiku Furikabutte
Saiyuki and all associated sequels and prequels.
Gohou Drug + sequel
Pet Shop of Horrors -- Very much so. Not the sequel, though.
Mahoutsukai no Neko
Shounen Hakaryuudo
Samurai Drive
Banana Fish
Kyou kara Ma no Tsuku Jiyuugyou! -- I wouldn't recommend the anime, because apparently it's out-and-out BL.
Waltz
Ilegenes and its sequel.
Yakumo Tatsu is a great one, too bad it's been dropped.
Sougiya Riddle
X and RG Veda -- The F/M romances are the predominant relationships in these two, but it is early CLAMP we're talking about here, so...
Yasha
xxxHoLic -- Very one-sided. Doumeki's devoted; Watanuki (unjustly, imo) thinks he's a nuisance. It somehow still fits, though.
Hakudatsu no Masquerade
Demon Diary
Butterfly -- If you don't mind them having quite the age gap. The kid is very mature, though.
Switch
Natsume Yuujinchou
Makai Ouji - Devils and Realist
Himitsu -- Is in scanlation limbo hell, unfortunately.
Kaizoku to Ningyo
Hetalia - Axis Powers -- But this is gag, so the bromance isn't actually serious, despite playing a pretty important role in the plot.
Servamp
Shirogane no Ou and its sequel, Shirogane no Ou - Fenrir Craft
Saraiya Goyou -- Also has an anime.
Ping Pong -- Though it's not really the focus.
Count Cain + God Child
Gin no Yuusha
RustBlaster and Yami no Matsuei -- Maybe. Being ages since I read these, so memory's a bit hazy.
Do Whatever You Want
Watashi no Kakurega e Douzo -- Only the last oneshot is BL, hence the genre tag. At least be sure to read Inhale and Exhale and World Market if you don't feel like reading the whole collection.
Game x Rush
Wild Adapter -- I'm pretty sure the series has been discontinued, but it's worth reading what's out for the excellent bromance.
Tactics
Monochrome Factor -- Well, sort of. The MC's too selfish to really have a sincere relationship with anyone except his sort-of GF, but since you mentioned Kuroshitsuji you should take a look at this, too.
Kaine -- The title story only.
And last but not least, most instalments of Jojo no Kimyou na Bouken which, despite the weird art and weirder storyline, is the best action + adventure + supernatural manga I've read.

Last edited by calstine at 10:47 am, Jun 2 2015

OHMHAAAYGOOODSSYEEEHS, you are simply my favorite person right now.