Archangel Seria is broken as shit. All permanents also includes Lands, Enchantments, and Artifacts, so depending on the progression of a game thi scan result in an instant 10 - 14 damage (at least) for just two mana, followed up by a devastating attack, since tapped creatures can't block. For blue red, this card seems way to creature-oriented.
Child of Nature doesn't fit blue either. I'd recommend making it green, giving its ability an additional cost of {1}{g}, and renaming it into something more creative.
Cipher is not expensive enough. Also It's name is shared by a keyworded mechanic, which can get confusing.
Death valley is crazy. Effects and abilities like this should never be on a land. Remember: They are FREE. Turn one, bang! The lifegaining-part doesn't fit red.
Dragon Rider needs some serious rework. I'd recommend making it 1/1, setting its bonus to +2/+0 and allowing you to search your deck for a dragon ONCE, when it enters the battlefield.
Energy Siphon... Just scrap the idea. You've to be careful with this kind of card. Also, it's cheap as fuck for only two.
Other than that Exalted Mermaid doesn't look very exalted at all, it might be necessary to increase it's cost to {2}{u}{u}, as well as {1}{u} for her ability.
Exalted Phoenix doesn't make much sense. Also it's not very much aligned to its color.
Gatherer of Wisdom is superstrong. Just don't make direct fetching for only two mana. It's sacrilegious.
Hell's Rider is too strong and not even very creative. It's just a 3/3 with TONS of keywords (some of which don't even match the color and flavor of the card). Especially its first ability is too strong.
Most of the rest are pretty bland and forgettable, especially the highland-cards. You keep mentioning the Highlands, but that's not enough to create Flavor...
My advice: Try to focus more on making fewer cards, with a higher standard of quality, especially regarding the flavor. Give us something to DIVE into, create a world, think it through.
Just try to work with more attention to detail. Maybe you should go through some existing cards to get a better hold of how the colors behave and the game works.