The mermaids were well known for their beauty and agelessness, meaning that their current physical appearance will never change. They stayed the same age ever since they were born. Even though they're a bit smaller than Loki now than in the past, the Jotun never criticizes their differences. The mermaids weren't born with names, so Loki actually called this mermaid "Syrena" when they first met when he was a boy. She and the prince recognized a quality in each other and they became allies. This wasn't love, but it was a romantic friendship. 

Syrena guided him into the blue-lighted cavern as the scales on her tail glowed in luminescence of the light. As the two mermaids were swimming by their side, humming their tune, their attention was mostly on Loki. Unlike Syrena, her sisters were far more aggressive after seeing a new arrival entering their territory. Since Syrena didn't trust them to be near him, she would always find Loki first at the surface. It was just for his safety. This didn't mean that Loki couldn't trust the mermaid kin, but he just needed to be careful when around them. 

As they reached the end of the tunnel, the next cavern was filled with pulsing blue vines that dangled from the ceiling. It looked like the lights were becoming bright, then dimming, and repeating. Syrena guided Loki through the vine-like cavern before stopping in front of one. She took his hand and guided it to one blue vine. "Touch", she said simply with softness before letting go of his hand, letting him touch it on his own. "Feel energy flow through you", she said as she stayed by his side.