After much searching in Adventure Mode, I have finally discovered how to unlock Princess Ruto in Hyrule Warriors. Princess Ruto should be one of the first characters that you unlock in Hyrule Warriors, if it weren’t for the fact that getting the Power Braces item takes a bit of grinding to get to. Nevertheless, the effort needed to get to Princess Ruto is worth it, as she is surprisingly powerful character, much better than the likes of Agitha. (My opinion of course)
Princess Ruto is located at the middle quadrant of the map. For your reference use the image above. I have marked the grid location by the red circle. Reaching to the point should not be too hard considering that it is so close from the initial starting point in Adventure Mode.
Press X to search the area. You will need to use one Power Bracelet on the green boulder to reveal the secret passage. Doing so will unlock the Princess Ruto reward in the challenge of the respective grid.
The challenge will restrict you to Darunia, so ensure that you at lease have respectable character built, with adequate badges invested and a high level (around level 25). This will greatly assist in making the challenge a little easier, particularly against the onslaught at the beginning.
Like with an other challenge, it is important to maintain control of the outposts and keeps that surround your Ally Keep. This will prevent your keep from getting overrun by enemy forces. In addition to that, controlling a large strategic position will grant you time to head out on your own to take enemy keep and further strengthening your position in the battlefield. As shown in the image above, I found controlling the west and middle keeps vital to swing the battle in my favor.
To earn victory you will need to defeat Princess Ruto at the North Keep. Please be mindful, that she is a tad elusive to hit, with her tidal wave attacks which does respectable damage to Darunia. Defeating Princess Ruto will end the mission and will unlock the Zoran princess. Congratulations! I hope you found this guide helpful. Be sure to check Nintendo News Fix Hyrule Warriors Character Guide.