If you are free-to-play player who has no intentions of spending real money on the game it is advised that you focus your attention on the characters that are easy to upgrade. The reason being is the 4-star and 5-star characters you summoned using the gems you were initially given at the start of game will not be easy to upgrade as the fragment drop rates for high-rank characters through Adventure and other modes are low. This means getting the fragments through the summoning procedure to upgrade these high-rank Skylanders will cost you thousands of gems.
As you play through Adventure mode there are some free 4-star natural characters you can unlock. These include Stealth Elf, Hot Dog, Blades, and Tuff Luck. Focus on upgrading these characters when you get them as you will be able to easily obtain their fragments by repeating the early stages. As the fragment drop in the early stages is good you will not have too much difficulty upgrading and powering-up those particular Skylanders.
Below are the stats and special abilities for Stealth Elf, Hot Dog, Blades, and Tuff Luck.
ABILITIES
Poison Spores
Fires a poisonous fog at the enemy with the highest ATK. Casts Poison for 3 turn(s) when dealing a critical hit.
Blade Slash
Slashes an enemy twice. A rampage which attacks all enemies will be activated with a bonus hit when dealing a critical hit.
Stealth
Gains Stealth for 1 turn.
Superior Stealth
Gains Stealth for 1 turn and is freed from all debuffs.
ABILITIES
Super Comet (Active)
Deals damage to an enemy. 60% chance to grant ATK UP effect for 1 turn(s) to an ally.
Ultra Comet (Active)
Deals damage to an enemy. 60% chance to grant ATK UP effect for 1 turn(s) to all allies.
Fire Bark (Active)
Spits out a fireball at an enemy. HEALS the ally with the lowest current HP when hit is clean.
Accurate Flame (Leader)
Increases ACC by 3% of all allies.
ABILITIES
Blade Shards (Active)
Throws a blade at an enemy. Decreases the target's Endurance.
Cyclone Swirl (Active)
Throws a blade at an enemy. Decreases Endurance. Also gains EVASION UP for 1 turn.
Wing Slice (Active)
Inflicts damage on an enemy and casts ACC DOWN for 2 turn(s). Also [removes 1 buff(s).
Fly Like The Wind (Passive)
Counterattacks when evading
ABILITIES
Traptanium Warblades
Slashes an enemy with double Traptanium blades.
Garden of Pain
Slashes blades at an enemy and casts DIMINISH for 5 turn(s).
Green Thumb
Jump onto an enemy and stabs them. HEALS herself if enemy has Debuffs.
Glave Wave
Launches a sword slash at an enemy. 70% chance to remove 1 buff(s).