He takes the appearance of a young man in his 20's. His attire consists of a military vest and a hood which covers his head and face. He stands at an above-average height.
He's a ruthless serial killer, and mercilessly chases his target, no matter who or where, he will continue to pursue them relentlessly until he succeeds on his murder. Only when he's at a serious disadvantage does he run away. He has also shown to be a coldly methodical and calculating individual, much in contrast to his original counterpart who used a simple method of cornering to murder, as Southpaw has been shown to dedicate a large deal of his time in fabricating several different and needless to say, well-crafted plans for assassinating his victims.
He also seems to be delusional as he talks to his already, dead mother. However behind this lies a broken lonely man, who has nobody but himself.
Diary & Abilities
He has The Murder Plan Diary. This potentially dangerous Diary allows Southpaw to calculate the chances of success of his tactics. This allows Southpaw to enter several different plans on his Diary and then being capable of performing the best course of action based on the success rate of each plan.
While the Diary can predict the success rate of each plan, a success rate as nearly perfect as 87% is unexpectedly not fail-safe. This is because while being the "best" course of action, it does not mean it will simply succeed smoothly, as he can in fact run into trouble, or unexpected circumstances that his Diary will not tell him about as it merely predicts a basic, general outcome seemingly ignoring potent changes in the Future. This means that even if he has a high chance of success, his Diary can also at the same time show him a Dead End entry.
He's a cunning strategist and a careful planner. He can determine when he's at a disadvantage and needs to run. Because his Diary needs plans to work, Southpaw invests his time in creating and simulating many different plans for murder, all of them are very detailed and elaborated. He is a fairly competent close-combat fighter as he has shown to fight for extended amounts of time using his signature ice pick, and even when battle against two, he was capable of giving a good battle.
He has also shown to be deadly in infiltration and was also capable of booby trapping the doors and windows of Yuno's mansion without being noticed, such as wiring the doorknobs to make it unable to open, or even electrify some of them, making any unfortunate victim unconscious from the shock. These acts are similar to those of Reisuke Houjou.
He was living alone with his mother that was very strict and hard on him, but he nonetheless respected. His mother eventually fell ill and decayed, prompting him to take care of her, although she was still the same towards him. She eventually died from Pneumonia 3 years before the start of the series.
The Survival Game
He later gets his Future Diary through unknown means and becomes aware of the Survival Game. Knowing that the other participants are carries of a special Smartphone from Chronus Seed Corporation, he begins a massive manhunt on each one, killing each owner one by one in the hopes of killing the participants.
He eventually corners and battles Arata Hoshino whom he pursues in the Planetarium of Sakurami City, but Yuno Furusaki interferes helping Arata. While Yuno urges Arata to run for his life, he eventually cannot bring himself to leave Yuno by herself and decides to fight Southpaw with a bat. He's initially successful in battling the mad killer, but just before Arata is about to be stabbed by his ice pick, his father Kurou Hoshino jumps in, and receives the mortal wound. Yuno uses this time to wound his arm with a knife, prompting him to retreat.
The Murder Plan
He talks to the corpse of his mother, elaborating new plans to kill Arata. And claims he has a good plan before departing. As he has loaded his full plans on his Diary, he booby traps Yuno's Mansion making it impossible for them to escape, and then starts calculating the success rate of each plan, from burning the house with a 27% success rate, to turn the lights off, which has a 87% success rate.
He then appears himself in the mansion unmasked, and claims he's there to kill them. Yuno tells him that both her and Arata's Diary predict Southpaw's death, but much to her shock new entries appear in both diaries that show that Arata is going to be killed. Southpaw reveals he also has a Dead End flag, and thus wonders who shall win, and he's ready to kill them, as he couldn't before, calling Kurou an interference. This infuriates Arata who battles the psychotic killer, the two engage both having a brief upper hand against one another, before Arata can throw him on the floor and tries to strangle him infuriated. Yuno accidentally drops a lamp and Arata comes to his senses, but Southpaw recovers. Yuno's Diary foretells Arata's death in the dining room so they leave. The two try to escape, but as Yuno is electrocuted by a booby-trapped doorknob, Southpaw appears and tells them that there is no escape. He rushes at Arata, but he uses a spray on his head which allows him to escape carrying Yuno.
Southpaw begins to look for Arata, as he looks on Southpaw from the second floor. Yuno awakes and is trapped in a room she tried to hide from Arata full of strange writings, which prompt a terrified Yuno to beg Arata to let her go. Southpaw sarcastically asks if it is fine to leave her there, as he had seen the contents of that room. He goes into detail that Yuno was locked there on her childhood with little to no food at all, with the only thing she could do was think on what to say to have her parents forgive her, further declaring that she will go crazy inside, which is soon proven right as Yuno screams in horror for forgiveness. As Southpaw begins to look for Arata, Arata rams him from behind and releases Yuno who had fainted. He carries her but is soon encountered by Southpaw, who goes into further detail about Yuno, knowing that her parents are dead and asks Arata what happens to a person like that. Based on his own experience he says that they become empty and desperate as they don't know how to survive on their own. Arata asks if he's the same as her and he says that he's not, as he will be able to create a perfect future for her mother.
Southpaw rushes at Arata who goes upstairs, and the two battle, but Southpaw manages to overpower Arata and throw him to the floor, as Arata's Diary is on his reach. He tries to smash it but Yuno wakes up and stops him, trying to run away from him, but he catches her and urges her to give up, but she states her resolve to protect Arata and brutally kicks him throwing him to the floor. Southpaw asks why she's willing to go that far for Arata, and Yuno berates him for thinking she and him are alike, and says that what Southpaw truly wanted was different parents whom he could lean on, and that he was running away from sad things, and that she's not empty as she overcame her difficulties and loves Arata.
Southpaw asks what's wrong with leaning on someone, as he did with Deus, and declares that personality is his use. Arata and Yuno come to a realization, that Deus had used Southpaw as a pawn on his game. Southpaw denies this saying that he promised him a bright future, but Arata revokes this saying that if that's is true then he hasn't done so. Horrified at the truth he reads his Diary, which changes his previous entries with others that reveal the truth he was blind to, that Deus deceived him. With no more will to live, he throws his Diary at Arata and urgues him to destroy it saying it is fine. Arata tries to do it, but hesitates remembering past events, and is unable to break the Diary. Arata claims that he won't forgive him but will not kill him either, giving his Diary back to Southpaw and encourages him to discover themselves what will happen if they defy their Dead End countdown, whether they die or a new future is created. However, in a last act of humanity, Southpaw pierces his own Diary, saying that Arata and Yuno are good people and asks them about people in coma (referring to his mother). He says that people in coma sleep and wonders what her mother dreamed about, before disappearing into thin air.