A confused companion

The morning was refreshing, and the group and I set out to complete a guild assignment: defeat some humanoid-shaped monsters that had appeared near a medicinal herb collection zone. I couldn't think of anything else but continuing to strengthen myself.
From a distance, I saw a child being chased by three monsters. I rushed to help him, but just as I was about to attack, a shadow swept in and took care of the monsters. The child ran towards Aria.
"Are you okay?" Aria asked in a soft tone to calm him down. "Y-yes," the child said, still scared.
When I turned my head, I saw the person who had saved the child. He wore a hood that covered his face, but I felt a great force emanating from him.
"Who are you?" I asked. "I... I know you!" he said with an anxious voice. "I've heard how strong you are! If I recall correctly, your name is Kaoru, right?"
I was confused by his enthusiasm upon meeting me. "Sorry... I haven't introduced myself yet. My name is Rei, and I'm a rank A adventurer," he said with a proud voice, revealing his face and showing a boy with blond hair that seemed like flames.
"Hello, I'm Aria, and I'm the..." "The beautiful Aria? Your name is known in almost every city!" he exclaimed with great enthusiasm.
"And... who are you people?" he asked, confused. "You don't know us, but you know them? We're Nara and Lysbeth, and we're part of Kaoru's group," Lysbeth said, slightly annoyed.
After that, Rei offered to accompany us, and Aria said it would be a good idea, so he joined us.
On the way to the assignment location, we encountered some monsters. Rei, wanting to demonstrate his skills, took care of them in an instant. When we arrived at the location, we found the monsters. Together, we defeated them all, although it wasn't much of a challenge since we defeated them easily.
I began to evaluate the area, searching for any clues as to why those monsters were there. Suddenly, I felt a presence approaching us. "Everyone, be alert!" I said with great haste.
A high-ranking monster emerged from the bushes: it had enormous claws, red eyes like the other monsters we had faced, but this one was different. It wore armor made of some animal's scales and a bright, fiery red sword, like hot metal.
Rei charged into attack without hesitation, but the monster struck him with a blow that sent him flying several meters back. He lay on the ground, badly injured. Aria rushed to cover him, while Nara used healing magic. Lysbeth and I jumped into attack, but the monster's armor scales were too resistant.
Seeing the problem we faced, I decided to retreat a few meters. Rei, looking at the monster, told us that it was the same monster that had destroyed his old village and killed his parents, and he had been training hard to avenge them ever since.
I was surprised to learn that there were others who had also had everything taken away from them. Filled with anger, I extended my hand to Rei. "Then... let's take revenge together."
Activating a new trick I had discovered the night before, my body was covered in dark magic that increased my strength and speed up to four times, but at the cost of consuming a lot of energy. Rei also became serious, covering himself with his hood as we prepared to attack.
"My vengeance is getting closer, I can feel it... and now, there's someone who shares that same desire for vengeance with me."