A build for a friend, but I'll still explain.

[Harrow], being just that cool, uses all of his abilities to power one another. Using 1, aimed towards enemies, charges your shield for use of your 2nd ability. Using 3 charges all of your abilities. Finally, using 4 saves your teammates from DBNO, or death in case of arbitrations. Since the 3rd ability can also provide energy to teammates, we must keep it at 100% range or higher.

Duration Mods - [Primed Continuity] (Can be replaced with Continuity if you don't have it) and Augur Message. (Helps with 2, 3, and 4 abilities)

Efficiency Mod - Streamline (Helps with all abilities)

Strength Mods - Augur Secrets, Power Drift, and Umbral Intensify (Helps with all abilities, most notably the 4th ability)

Shield Mods - [Redirection] and [Augur Accord] (Helps with the 2nd ability)

The best ability out of his whole kit is his 3rd ability, which can provide up to, if you rank up whilst using it, 184 energy per kill. This is very good for frames like [Titania] and [Mesa].

That's all from me, comment if you need any help. <3