When you sign up, you'll receive your first 8 Characters free of charge. At each level, a character is defined by its Power (P) and Damage (D). Some characters also get a special ability when they reach a certain level.
All of this information is indicated on the character's card.
(1) Character's name - Self explained.
(2) Character's level/star count - Self explained
(3) Illustration - The card's art.
(4) Power - Power is also known as attack. If you have, in this case, 2 power, then for every pill you use, it will add 2 attack.
(5) Damage - Damage is how many life points you will take away from the opponent.
(8) Clan Logo - The picture of the Clan the character is in.
Cards get XP which is then used automatically to level up cards to their final level. The ammount of XP needed per level is shown below.
Level 1 - 2: 500xp
Level 2 - 3: 1500xp
Level 3 - 4: 3000xp
Level 4 - 5: 5000xp