Our friends at Card Gamer also have an article listing some of the most expensive Digimon cards, should you want even more detail
Soulless One originally had some black ability (regeneration, I believe), but the development thought it would be cooler if its extra ability was that it counted zombies in play and in the graveyard
Material Save 1 (When this Digimon would be deleted, you may place 1 card in this Digimon's DigiXros requirements from this Digimon's digivolution cards under 1 of your Tamers.) [Special Play Condition] DigiXros -2: 1 Digimon card w/[Xros Heart] in traits When you would play this card, you may place specified cards from your hand/battle area under it
The saga isnt over though, because Rydia, Summoner of Mist is another fantastic two-drop
More interesting is an upcoming Community Week event, which will give players new accessories and hourglasses if they share and trade cards with friends
How about trying Arcbound Whelp, Dragon Whelp, Fledgling Dragon, Firescreamer, Furnace Whelp, Henchfiend of Ukor, Lightning Dragon, or any other firebreathing options at four mana instead