This is because the other four colored mana symbols are in its rules text for its activated ability
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Tale of Momo's first ability)
Being the most available party member, it may also be beneficial to train him in healing jobs, such as White Mage, Blue Mage, Red Mage, or the Chemist
None of the other cards necessarily provide you with a cards worth of value, and both Knight of Old Benalia and Faerie Seer arent that impressive to blink (the Faerie is much better when youre in UB Ninjas)
You can activate its last ability, but it won't do anything