Shinra Kusakabe is a third and fourth generation pyrokinetic, meaning, he is able to create and control fire on his own. He is also one of the few third generation pyrokinetics to be in possession of the Adolla Burst, the flames of perdition that originate from an alternative dimension. His ignition ability allows him to emit and control flames from his feet. Using his ignition he can move around at high speeds and use scorching kicks to take down his opponents. 

To answer the question about how Shinra gained his powers shortly;

Shinra Kusakabe awakened his third generation pyrokinetic powers in the fire accident that was caused due to his little brother, Sho Kusakabe, activating his Adolla Burst 12 years ago. Shinra’s flames were initially considered to be weak, but he later discovers that he too possesses the Adolla Burst.

He was later termed to be a fourth generation pyrokinetic by Haumea, when he was able to unlock the powers of his Adolla Burst, by temporarily receiving the Evangelists grace via his forced Adolla link to Sho Kusakabe.

Shinra has often been held responsible for the fire accident that apparently killed his mother and brother. However, after his encounter with the Evangelist’s henchmen, Leonard Burns reveals to Shinra that it was Sho who was behind the fire.

How did Shinra Kusakabe activate his third generation powers:

In the Fire Force universe, the pyrokinetic abilities of people are closely related to the phenomenon of Spontaneous Human Combustion (SHC). Depending on how a person’s body reacts to the SHC, they are categorized into the generations. However, there are some humans who have developed a natural resistance or adapted to these flames. The Fire Force is made up of such personnel whose bodies adapted to SHC and awakened pyrokinetic abilities. These personnel are then able to manipulate (second generation) or produce flames (third generation pyrokinetics) according to their will.

A similar SHC was the trigger that awakened Shinra’s third generation powers. This SHC was the cause of the fire accident that burned down his home in the year 186 (when Shinra was 5 years old).

On the night of the accident, Sho was the first person to undergo SHC and activate his Adolla Burst. The effects of the Adolla burst caused his mother and Shinra to also be victims of the SHC. Shinra’s mom had no special abilities to control or manipulate the flames, so she turned into an infernal demon(first generation). Shinra mistakes his mother’s infernal demon to be behind the fire. Shinra however adapts to the flames instead of being consumed by it, thanks to the influence of Sho’s Adolla Burst and activates his ignition ability.

His flames were initially considered to be weak and he had no control over them. The Skill Development Facility at Hajima industries, where Shinra used to stay as a kid, carried out a lot of tests on him. However, they were not able to confirm if Shinra possessed the Adolla burst or not.

How did Shinra awaken the full powers of Adolla Burst:

Shinra probably had the Adolla Burst in him since the day his powers were activated. However, the full abilities of Shinra’s Adolla burst were unlocked during his face off with Sho Kusakabe, making him a fourth generation pyrokinetic. 

In the fight, he temporarily receives the Evangelists grace, via a forced Adolla Link he formed with Sho. This allows him to travel at light speeds, causing his body to split into subatomic particles in the process. When these particles surpass the speed of light, they travel back in time and his body reverts to the point before it broke down.

Shinra and other people possessing the Adolla Burst are currently desired by the Evangelist and the White-clads in order to facilitate their world wide cataclysm once again.