Laghu Parashari states that any planet gives its special fruits according to its own nature only during the antardasgaa of its "sambandhI" (relatives) and "nija-sadharminaH" , while during the antardashaa of itself under its own mahadasha a planet gives only general fruits and not its own specific fruits.
Four-fold RELATIONS are explained in detailed in Laghu Parashari, and BPHS also mentions these relations in scattered verses. But Sadharmi is not specifically defined because it was deemed to be too simple to be defined separately.
Any two planets are sadharmi if their astrological fruits (depending upon their characteristics, nature) are similar. For instance, if two planets are killers, they are sadharmi. In that case, the killer fruit of planet-A in its mahadashaa will be given during the antardashaa of planet-B.
Therefore, sadharmi should not be defined on the basis of any single trait. It is the quality of fruitsof coming from a planet. Suppose planet-A is killer but it has many other benefic traits also. Then, during its own MD, it will give killer fruit only during the AD of its killer sambandhi or killer sadharmi, while the same planet-A will give benefic result during the AD of benefic relatives or benefic sadharmis.