Ernst

What we know

Loyal aide and servant to Lady Fiona Wachter in Vallaki (Session 11 @ 00:46:33)
Scrawny man in long trench coat and plain hat (Session 11 @ 00:39:09)
Tailed party near town square; halted by Sildir's Command (Session 11 @ 00:39:52)
Delivered sealed invitation to Saturno and friends for dinner at Wachterhaus tomorrow 6 p.m. (Session 11 @ 00:44:17)
Reported Saturno's shoe incident against Izek to Fiona immediately (Session 11 @ 00:46:00)
Drew rough Vallaki map pointing to north-side Wachterhaus (Session 11 @ 00:48:45)
Urged formal attire; noted party's post-combat dishevelment (Session 11 @ 00:47:59)
Refused nickname "Ernie" (Session 11 @ 00:46:52)
Delivered Izek daily schedule and poisoned Red Dragon Crush at Blue Water Inn breakfast (Session 15 @ 00:52:09)
Tailed Izek for days; coup moves day after kill (Session 15 @ 01:03:10)
~24 Vallaki guards; 12 on duty; Izek's escorts vary daily (Session 15 @ 01:04:03)
Held umbrella over Lady Fiona Wachter on coup stage 9th (Session 20 @ 00:11:47)
Delivered 100 gp sack to Mercurio as Fiona's promised payment (Session 20 @ 00:40:51)
Helping Ireena Kolyana organize refugee wagons to Barovia Village 9th evening (Session 21 @ 01:08:27)

Alley confrontation after party detected him following them during Session 11 (Session 11 @ 00:41:08).

Unconfirmed: whether Ernst knows party also met Baron Vargas Vallakovich same day (Session 11 @ 01:27:46)