I'm in love with this game! I have never played something like this before, it's refreshing! But I have a question, why Porygon2 is not in the game? It's a Gen2 evolution just like Crobat and I don't understand why it is not there, I have more questions like what are those cards with different colors? Are those the legendary cards I've seen people talk about? And, is there going to be more events in the future like: "You have encountered Team Rocket Grunt! if you lose this battle you will lose one of your deck cards, but next time you fight another grunt and win the battle you will regain your card/s back" also it would be nice that they more money than intended when losing, also having other types of events would be nice! (Also I believe this is a bug but I can't level up my porygon to level 3)
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Your porygon isn't bugged you just used it for a tribute to boost another pokemon you can see the x at the top of the card and the heart. A card can have up to 3 hearts.
Because of the x you cant do that mucht with your porygon like evolution or level up, but you can tribute another pokemon to your porygon so it will be back to normal. Maybe the picture helps
Before Tribute
You had a good question so i made a quick research for you maybe it helps.
There must be a intern multiplier wich will be increased with each heart at the top.
At 3 Hearts the multiplier is round about 1.4 so u can multiply atk/def/life with this 1.4 and can calculate how strong the pokemon will with 3 hearts.
The multiplier is not exactly 1.4 in this sheet maybe because of rounding stuff only the owner knows the truth.