You can obtain more heroes by spending Gems in the Summoning circle in the main lobby. You can get Gems from codes, daily quests, AFK Training Dojo, and clearing Story Worlds.
Yes. Every banner guarantees a Legendary hero within 100 summons. However, pity does not carry over between different banners.
You can raise your heroes' star levels by fusing them with duplicate heroes of the same star rarity, or by using Fusion Crystals.
Level caps depend on the hero's rarity. The absolute highest tier characters can currently reach level 340 (Eternal quality).