Like his namesake, he appears invincible, always getting back on his feet after taking a severe beating. This is actually due to his fighting style which uses counters and dodging an opponent's attack by the slightest margin. As such, he usually falls down on his own to fool his opponents. He also names most of his attacks after musical terms.
Nevertheless, the Go no Sen has a critical weakness: he fights solely based on counters and is unable to attack unless he is attacked first. This is shown when he rushes at Fortuna, only to stop because Fortuna didn't attack him. He generally tries to cover this weakness by releasing large amounts of killing intent as he rushes at his enemy to cause them to attack him instinctively.
Of the former Ragnarok members, Siegfried has probably become the most powerful besides perhaps Natsu Tanimoto.
Abilities:
Acrobatic Skill: Most of Siegfried's attacks involve him rotating rapidly to accumulate both attacking and defensive power.
Imitation: Dressed as Hermit as part of a plan to induce the school idol into Shinpaku Alliance, Siegfried is able to effectively imitate Hermit's movements and defeat common Ragnarok grunts while maintaining the disguise
Enhanced Endurance: Siegfried's physical fortitude is great enough to enable him to fight against the likes of Berserker for a decent amount of time. He has also shown to fall from extremely great heights and land with taking little to no damage.
Enhanced Speed: The training that Siegfried received from Tibet enhanced his speed to the point where he can traverse across several meters at high speed, as seen when he intercepted the attack on Ukita after Niijima finally gave an order to defend the former.
Martial Arts (See Below)
Techniques:
Nirvana No Canon: Siegfried's basic technique. He performs a pirouette on one foot, with his arms aloft and hands positioned outwards by his sides, using the rotationary force to either defend from attacks, or to enhance the force of his blows.
Nirvana No Canon, Vertical: Similar to the above technique, but instead performed while cartwheeling in the air.
Senpu no Da Capo: Siegfried first falls backwards, then spreads out his legs in a windmill kick, tucks one leg in to strike with the other, falls back into a thrust kick aimed towards him, and finally stands up and goes into a back kick.
Requiem Buster: Siegfried latches onto his opponent form behind before jumping up into the air and piledriving them headfirst into the ground.
Tenshi No Sasayaki (Whispering of the Angel): A close-combat technique that Sieg perfected in Tibet where, after locking his body around a target (or vice-versa) in a position where his head is right next to the victim's, Sieg begins to inhale large amounts of air through his mouth, causing his mid-section to swell up dramatically. Then after clearing his throat like an opera singer, Sieg would lean in close to one of the ears of the victim to yell a deafening (along with a gust of wind that actually goes through the ear and passes through the other at the other side) powerful enough to be heard throughout an entire coliseum, effectively disorienting the victim into unconsciousness.
Meikai no Waltz (Waltz of Hades): Siegfried grabs an opponent from behind and drives his knees into his opponent's side.