What is ICMPv6 and what extra roles does it take on compared to IPv4's ICMP?

ICMPv6 is the redesigned control/error-message protocol for IPv6; beyond classic error messages it adds "Packet Too Big" and absorbs multicast group management and Neighbor Discovery.

ICMP in IPv4 mostly did errors and ping. ICMPv6 is far more central to how IPv6 works:

• New/important message type: "Packet Too Big" — the backbone of Path MTU Discovery (since routers no longer fragment)
• Multicast group functions (the role IGMP played in IPv4) are folded into ICMPv6
• Neighbor Discovery Protocol (Router/Neighbor Solicitation & Advertisement) — the replacement for ARP — is built on ICMPv6 messages

Because so much depends on it, blocking all ICMPv6 at a firewall breaks IPv6 (unlike IPv4, where you can often filter ICMP fairly aggressively).

Tip: ICMPv6 uses Next Header value 58. Don't firewall it blindly — Neighbor Discovery and PMTUD will stop working.

Go deeper:

• ICMPv6 (Wikipedia) — the message-type table, including "Packet Too Big" and the NDP messages that ride on ICMPv6.