The lion is the guardian of the portal I think and he claimed sena as his true master. This was I believed in the first season when he said that she can call him if she ever needs him. I might be wrong and it might be in the second season. But the necklace that sena has on is a mana stone which is like a key to open the portal. Which is also where the lion lives in.
Ty qweety
Sweety* i had completely forgotten about that
Yea the necklace is indeed a key cuz our ml had one of the mana stone in the first season that he took from another saints to open the portal without her being there but after he accidentally met sena during the late night when sena first open the portal. Sena absorbed the mana stone that the ML had and now he no longer can open the portal thanks to her. While sena can easily open it with or without that stone but it's just easier to open the portal with da stone is all.
Bro wtf since when does she have a magical lion and did he get out of her jewellery ???