Who pairs well with Ace?

Thatcher: Disrupt, then breach. Kali: Lance, then breach. Twitch: Zap, then breach. Thermite: Two breach points.

Ace: counters, combos and tips

Q: Who counters Ace in Rainbow Six Siege?

Bandit, Kaid, Jäger, Wamai and Maestro. Bandit: electrified wall destroys/blocks the S.E.L.M.A. Kaid: Electroclaw on the wall or hatch blocks it. Denies Ace's breach. Jäger: ADS catches the thrown charge.

Q: What does Ace counter?

Mira and Mute. Mira: breaching the mirror wall destroys the Black Mirror. Mute: jammer does not stop the S.E.L.M.A., so Mute alone cannot keep a wall shut against Ace.

Q: What is Ace's gadget?

S.E.L.M.A. Aqua Breacher. Thrown hydraulic hard-breach charge (3): sticks to a breachable or reinforced surface and opens it in two ticks on walls, one tick on floors/hatches.

Attacker · Nighthaven · hard breacher, vertical (reinforced hatch)

Ace is countered by Bandit (electrified wall destroys/blocks the S.E.L.M.A), Kaid (electroclaw on the wall or hatch blocks it) and Jäger (ADS catches the thrown charge). Ace counters Mira and Mute. Ace pairs best with Thatcher, Kali and Twitch.

Ace's gadget: S.E.L.M.A. Aqua Breacher

Thrown hydraulic hard-breach charge (3): sticks to a breachable or reinforced surface and opens it in two ticks on walls, one tick on floors/hatches.

Who counters Ace?

BanditHard counterelectrified wall destroys/blocks the S.E.L.M.A.
KaidHard counterElectroclaw on the wall or hatch blocks it. Denies Ace's breach.
JägerStrongADS catches the thrown charge.
WamaiStrongMag-NET catches the thrown charge. Denies Ace's breach.
MaestroSituationalEvil Eye laser can shoot the charge if it has a sightline (usually only after the first tick).

What does Ace counter?

MiraStrongbreaching the mirror wall destroys the Black Mirror.
MuteStrongjammer does not stop the S.E.L.M.A., so Mute alone cannot keep a wall shut against Ace.

Best Ace combos

ThatcherDisrupt, then breach.
KaliLance, then breach.
TwitchZap, then breach.
ThermiteTwo breach points.
OsaOpen the wall together.

Where Ace is strong

Ace the bathroom wall after pre-firing gym rack for cross control. Sites: Clubhouse Cash Room / CCTV Room.
Strong breacher.
Strong pick on Bank. Sites: Bank Lockers / CCTV Room, Bank Staff Room / Open Area, Bank Tellers' Office / Archives, Bank Executive Lounge / CEO Office.

Ace myths

Mute counters Ace. The jammer explicitly does not stop S.E.L.M.A.; the player claim 'Mute can't jam Ace' is TRUE.

Key Ace facts (Y11S3)

Mute's Signal Disruptor does NOT stop the S.E.L.M.A. (listed among gadgets the jammer will not stop). (since Y5S3 (Steel Wave), unchanged)
S.E.L.M.A.s do not work on walls electrified by Bandit's Shock Wire or Kaid's Electroclaw. (since Y5S3)
Thrown S.E.L.M.A.s can be intercepted by Jäger's ADS and Wamai's Mag-NET. (since Y5S3)
S.E.L.M.A.s can be shot and destroyed even mid-sequence (so anyone with a sightline, incl. Maestro's Evil Eye once the first tick has opened the wall, can stop the second tick). (since Y5S3)
Two S.E.L.M.A.s open a reinforced hatch (one tick each on floors). (since Y5S3)
S.E.L.M.A. destroys Mira's Black Mirror if any part of the charge touches it. (since Update 6.1.3 (May 2021))

Sources

More attackers

Amaru Ash Blackbeard Blitz Brava Buck Capitão Deimos Dokkaebi Finka Flores Fuze Glaz Gridlock Grim Hibana Iana IQ Jackal Kali Lion Maverick Montagne Nøkk Nomad Osa Ram Rauora Sens Sledge Solid Snake Striker Thatcher Thermite Twitch Ying Zero Zofia