Grandwalkers are a type of Nexus which debuted in SD43 and SD44. While on the field, their own text stipulates that nothing except an effect which specifically targets a Grandwalker Nexus on the field can affect it.
Grandwalkers have 3 abilities unique to their type of card: Core Charge, Grand Skill and Grand Field. Each Grandwalker will always have a Core Charge effect and a combinations of a Grand Skill and Grand Field or two Grand Skills.
The text on a Grandwalker will always contain this portion:
Core Charge: (x/x/x and Cost x or higher)"
◆When you Summon/Advent a target Spirit/Ultimate, put one core from the void onto this Nexus.
◆When this Nexus is deployed, if you do not control a Nexus with the same card name as this card, send the top three cards of your deck to the trash. For each target card among them, place one core from the void onto this Nexus.
The core placed onto a Grandwalker Nexus cannot be used as core from a regular nexus would be used, and core from the reserve/spirit cannot be placed onto a Grandwalker Nexus as you would on a regular nexus unless the effect stipulates that you are able to do so.
When using a Grand Skill, the effect will require a cost of returning core from the Grandwalker back to the Void in order to trigger, whereas the Grand Field will remain active as long as the number of core on it meets the requirement for it to reach Level 2.