Galdino, aka Mr. 3
Warning; this may be slightly bias as Mr. 3 is by far my favourite One Piece character outside MAYBE Usopp. It's close.
What is so great about Mr. 3?
Well…
LOVE
I'm sure lots will agree but Mr. 3 has undergone one of the most startling character changes through the series.
Mr. 3 started the series as a no nonsense, sadistic psychopathic assassin who balked at the very idea of alliances. When Mr. 5 and Miss Valentine show up he threatens to kill them if they fail their mission again. He laughs at the stupidity of Dorry and Broggy's battle, and mocks Luffy's intelligence throughout the entire battle. Mr. 3 shows he is in Baroque works, ENTIRELY for himself. Even after the Alabasta arc he flat out refuses to help his former partner Miss Goldenweek, the person he should be closest too, rescue their captured allies and had it not been for Bon Chan LITERALLY forcing him to get involved, Mr. 3 would not have gone through this startling transformation by getting him sent to prison with him.
Fast forward to Impel Down and something's changed. Mr. 3 has been humiliated, beaten, broken down. By the time Luffy breaks in, Mr. 3 see's his opportunity to escape using Luffy and Buggy's ruckus. However unlike his past self he doesn't get mad that Luffy doesn't recognise him at all unlike most villains. It's not important.
He still plans to sacrifice Luffy to escape himself, but in doing so he forms his first real friendship in Buggy.
Along the way, he and Buggy become firm friends, with Mr. 3 keeping Buggy out of danger showing concern for someone other than himself. During their prison break antics, despite his better judgement he even frees the man who is solely responsible for putting him in prison in Mr. 2.
Cue wacky antics as frankly the best double double act in the whole series falls floor to floor causing chaos and hilarity in their wake. Mr. 3 and Buggy then cease their opportunity to escape and if we use the anime filler as well, it's clear that their bonds strengthened through their trials and ordeals.
Sometime later during the entire escape, Mr. 3 suddenly does something he's never done before. He can no longer just flee. He stands up to Magellan of all people blocking the sure kill poison attacks that not even Ivankov, Inazuma or Luffy himself have managed to resist. With this, he shows the first real piece of courage he's ever showed…but it won't be the last.
Bon Chan's sacrifice has an unexpected effect on Galdino. He reflects on his comrade and realises that maybe he wasn't such an awful guy. It's around this time I'd suggest that Mr. 3 really reflected upon his own actions, and with his new found power of friendship, he sets off to the greatest war in One Piece history alongside his bestest buddy Buggy.
Mr. 3 uses his past experience in spy work and assassination to neutralise a executioner before taking his place. Mr. 3 sets out to save Ace climbing the execution platform right under Sengoku and Garp's nose. We've come such a long way from Little Garden Mr. 3 now. Galdino still is wary of being officially friends with Luffy, but he has enough respect for both Luffy and Bon to risk his life to save a man he's never met before.
This character transformation alone is why I believe Galdino to be one of the most underrated One Piece characters in the series.
Speaking of; his friendhship with Buggy!
As previously stated, I found the two's antics to be a much needed breath of levity within an arc where we were beginning to miss the wacky Strawhat quirks. I'm so glad Oda decided to let Mr. 3 join Buggy's crew. This is one friendship I really don't want to see ever end. Mr. 3's straight man to Buggy's outlandish buffoonery is something that never truly grows old.
His devil fruit power and use.
As we've discovered through the series, it's not always about what fruit you obtain but how you apply it. Mr. 3 has used his devil fruit exceedingly well. From blocking attacks from Magellan and EVEN Sengoku to creating wax mechas, walls that can block attacks and giant knives and even turning birthday cakes into a trap deadly enough to make Zoro decide legs were overrated. He's also the only paramecia type to date that has a boat powered by his abilities.
His tenacity.
Mr. 3 gets punched out by Luffy. He's a bit fragile without his wax so we assume he's down for the count, but he gets back up, creates a wax mecha(which is awesome; just ask Luffy) and continues the fight. He eventually loses, and gets taken out with a singular kick. Seems a bit weaksauce right? Well, he then manages to sneakily avoid the assassins sent to kill him, sneak to Alabasta and greet Croc in person. Croc is not pleased! He drains the moisture from Galdino's body and throws him into the Banadile tank where he's eaten. Mr. 3 simply encases himself in wax stopping himself becoming a tasty treat and drinks his fill the moment he's free. He gets knocked out by Sanji. Devil fruit rules don't apply to him! He floats in the water. SCREW THE RULES. MY HAIR IS A 3.
His entire adventure with Buggy is filled with knock backs, but the two just keep going.
He manages to turn the tide!
Both in Impel Down and Marineford Mr. 3 manages to do one simple act each time that turns the battle in his allies favour. Applying his smarts, his spy experience and his creative use with his devil fruit, Mr. 3, for such an understated guy tends to make a big impact on the plot.
CONS
At first he's a rather paint by numbers insufferable genius type jerkass in Little Garden.
Rather an understated character. Despite his major contributions to saving Ace(he pretty much IS the guy who saves Ace) he tends to get ignored by things like Episode of Sabo, Burning Blood, etc. This is obviously a personal problem, but goddamn. He does tend to get overshadowed and outshined by anyone he's with; Luffy, Buggy and Bon for example are far bigger characters than he is, and far more impactful as individuals. In a way this is kinda clever since he's an assassin and standing out isn't something he should be doing.
He's rather weak willed.
Considering he's such a tenacious individual he's shown to not be able to withstand Luffy's conqueror Haki, and to date is the only named reoccuring character incapable of standing upto it.
Overly reliant on his devil fruit.
Unlike Bon, Galdino is very much reliant on his wax abilities. Without them he is essentially defenseless, which leaves him ridiculously unarmed whenever he comes into extreme heat, and with everyone under the sun learning fire based attacks as the series goes on Galdino's super useful abilities become less handy as time goes on.
Responsible for making the giants look like fodder.
Mr. 3 took down the giant leaders Dorry and Broggy…giants were then used as fodder until timeskip where Harujadin did something surprising me frankly.
Overall:
Character and Actions: 5/5
A villain turned acquaintance turned ally done right. Mr. 3's leap from evil to good has felt a little more natural than say…Hachi or Bellamy. Action wise he's saved the day twice using his devil fruit abilities at two vital parts of the plot.
Look and Design: 3(ha)/5
His hair is a set up for a gag about not standing out and revealing his identity. That's humourous, but aside from that…well...Galdino is just a rather regular guy outside his topknot. Which in a way is perfect for spy and assassination work, but it does mean he's not got the most outlandish or dynamic design in One Piece.
Impel Down onwards though give the guy an additional point because his beard game is on point.