(S) If there are no monsters on the field, you can Normal Summon this card from your hand as a Level 2 monster. (U) If your opponent controls a monster and you control no monsters, you can Special Summon this card from your hand as a Level 4 monster.
The (U) Unclassified summon effect does not start a Chain.
"Skill Drain" has no effect on the Level of "Level Warrior", because "Level Warrior" is considered to a already be on the field with the appropriate level. So "Skill Drain" will not cause the Level of "Level Warrior" to become 3.
Even if "Level Warrior" becomes Level 5 or 6 due to "Level Conversion Lab", you can still Summon it as a Level 2 or 4 monster with its effect.