Nero is a new character in the main story, first appearing at the end of Chapter 2. He and his partner (Rossa) lead Fisica and Ritmo through the Tunnel of the National Institute of Notalium in Chapter 3.
Nero has black eyes and a relatively slim figure, being much taller than Fisica and Ritmo. He has straight black hair that roughly reaches his diaphragm. He wears a white lab coat with blue trimming on the edges. Like Fisica and Ritmo, he wears a Notalium pendant to protect him from the effects of Al Niente. He also wears jeans that are tucked into his boots, which are standard brown hiking boots with cross-stitched laces.
Nero is shown to be very intelligent with a keen sense of danger. Immediately after tuning the Percussion Instruments in Androgynos A, he says that "this is not a place to linger around. Let's get out of here as soon as possible," showing that he understands how dangerous the Tunnel can be.
He isn't particularly friendly toward Fisica and Ritmo at first. Before Fisica questions his motivations and Rossa's in Chapter 3, Section 4, he says almost nothing to them.
Like Rossa, he's also a very reserved person, rarely voicing his emotions. He also tends to speak only purely factual information, and keeps quiet during emotionally tense situations. The most emotion he shows is when warning the group of the impending danger in Chapter 3, Section 5 (see quote in first paragraph). Even then, he keeps calm.
Nero seems to have a clear, defined limit for how serious he should be in a situation. When Fisica asks who he and Rossa are at the start of Chapter 3, Rossa telling her "it does not matter who we are", he chimes in with "her name is Rossa". This could mean that he doesn't like being totally reserved in moments like this, and would lighten the mood himself.
Rossa is Nero's partner in the Silence Border Guard. The two seem to get along somewhat, as we don't seem them arguing with each other.
Fisica and Ritmo Edit
Nero has only known Fisica and Ritmo since the end of Chapter 2. Because of that, he doesn't seem to be too warm towards them - but he doesn't seem to hate them, either. Nero also seems willing to tell the pair some things, like how he's part of the Silence Border Guard and how serious it is to open a seal of the Apocalypse.