Who are they?

2 Years of great adventures have passed for little April and her Teruvoo Echo. Together they have explored the forest in which they met each other. Defeated the evil monsters under the bed and in the closet and they are still not done helping those who are in need of a great hero to save the day. (Like the old women living a few houses away who always seems to need help with her groceries when April is around. Probably just a coincidence.)
With the help of Echo she even managed to create her own superhero outfit. It's not looking like it has been made by a professional but it's made with a lot of love and effort and that's what counts, right?
And after a long Day of playing Games with Echo she always returned home fast so her parents didn't have to worry about her.
Suddenly while April was drawing a picture of Echo and herself in the living room it knocked on the door and Aprils mother went there. Being all curious April followed her mother to see who has come to visit them.
On the doorstep were a boy and a girl introducing themself as their new neighbours. They claimed to live in the old abandoned mansion soon because it will get renovated.
"Both of you look pretty young to live alone in the old mansion, don't you?" Aprils mother asked. The two nodded and quickly tried to change the subject. "Yes you are right. But it is as it is now. A nice little daughter you have there. I saw her helping an old lady a few hours ago." the girl said. With a big smile Aprils mother proudly held on to April. "Yes, she is my little Angel." she said while raising her voice.
"She sure is..." the girl quietly whispered while you could see a faint smile on her face for a second. "Okay, well. It was nice to meet both of you. We will probably see each other again soon." The Boy who spoke for the first time added. "I'm glad too. Maybe you two would like to come over for Cake and Tea another day?" Aprils Mother kindly asked. The Boy and the girl accepted the offer with a nod and went away.
April could have sworn she had seen a creature hiding in the shadows before her mother closed the door but when she quickly reopened it the boy, the girl and the creature in the shadows were gone. All of them.
That was when April ran into her room to tell Echo about everything that just happened. To make sure they would investigate the case the next morning.