The Arrow of Strength is Pit's ammunition for his Bow. As Pit defeats enemies and proceeds through his quest, he'll occasionally come across Sacred Chambers. If he's gained enough experience (see below), Zeus (or a "friendly god") will bestow an additional level of strength to the angel, represented in the form of an arrow. Pit's hair will change color with each upgrade, and an additional arrow will show in the status screen. There are five levels of strength, each with their own arrow:
- Level 2: Bronze Arrows.
- Level 3: Silver Arrows.
- Level 4: Gold Arrows.
- Level 5: Sacred Arrows.
The power of Pit's attack is the same as his strength level. Thus, a Pit with a Level 3 strength can kill a Komayto in one hit, since it has 3 HP, but Uranos will require 2 hits, since it has 4 HP. Note that the Arrow of Light does not increase attack power; it only adds a piercing effect to each shot.
The means to increase these levels is a little obscure, as the game tracks a hidden stat that governs over the strength upgrade, but it functions alongside the Score. This hidden stat doesn't have an official name, but we'll refer to it as "skill" here. Pit is award a Strength arrow if this skill stat equals or exceeds 10,000 points by the time he reaches a Sacred Chamber. This total is reset to 0 at the beginning of a new level, except when restarting the game after defeating Medusa. Pit's skill cannot be reduced below 0.
Pit gains the same amount of skill as he does Score (experience) when killing enemies (100 for a 1 Heart monster, 300 for a Half Heart, 500 for a Big Heart), plus the same amount if collects their Hearts afterwards. However, even though Pit doesn't gain experience when killing enemies in Enemy Lairs, their Hearts count towards the total; this goes for any enemy that doesn't give experience. If Pit has the maximum of 999 Hearts, collecting more will not add to the skill total. Pit also gains 300 skill upon entering a chamber, including the stage exit (which means he really only needs to reach 9700 skill to achieve the upgrade). Pit loses 10 skill for each arrow shot and 300 for each hit taken, regardless of damage. In a Treasure Chamber, Pit also earns 100 skill for each Mallet collected, but loses 500 for each pitcher shot.
The exact mechanics for the upgrades in Kid Icarus: Of Myths and Monsters is unknown, but works in a similar fashion. Additionally, if the arrow's strength is over the HP of the enemy it hits, the arrow will continue on its path, piercing through multiple targets. Also, Pit can obtain more than 5 arrows.
Upgrade information generously used with permission from Daniel Remar's Kid Icarus FAQ hosted on GameFAQs, with a special nod towards Disch Wersher. You can find it here: http://www.gamefaqs.com/console/nes/file/587380/17485