Shounen Onmyouji, Tegamibachi Reverse Recommendations
Both main characters are little boys with big ambitions and a lot of heart. Both are always going out of their way to help others and care a lot about the people who support them. Both boys also have to learn, from square one, how to take care of themselves and fight monsters/spirits.
Mokkun (SO) and Niche (TB) also share the Pet/Protector element. If you like Tegami Bachi Reverse, I think you'll love Shounen Onmyouji!