Jääkäri
As everybody knows, the word comes from the German word "Jäger", meaning hunter (the Germand word; Finnish word Jääkäri doens't have any additional meaning"). Centuries ago, at the time of line tactics and muskets, jaegers were light infantrymen, who fought as skirmishers. During the WW2 Jaeger units in Finland were fast-moving bicycle troops, which were especially used to pursuit or delay the enemy or in recce duties. This varies though: each battalion of infantry had a separate jaeger platoon, which functioned as a scouts. The Jaeger battalions were somewhat closer to the regular infantry.
Sissi can be translated as either ranger or guerilla. I would prefer the first one, because the Finnish sissi troops were regular army forces, not some irregular bunch of civilians. They were more commando-type units.